billsplit: adopt the shared palette, retire the teal seed

Last of the four drifted apps. The theme was ColorScheme.fromSeed(teal)
plus fifteen ad hoc colour literals that mostly bypassed it -- not one
widget outside main.dart read colorScheme at all, so the seed barely
affected what you saw. It now uses design_system-v0.1.0's
buildLightTheme(), and the literals that have a real theme role read it
off the scheme.

Light, not dark, deliberately: the receipt screens carry black-on-white
helper text, so inverting brightness would make it unreadable. The
Colors.black54 sites are exactly those, now onSurfaceVariant.

The error family maps cleanly -- Colors.red to error, red.shade50 to
errorContainer with onErrorContainer text on it, and the swipe-to-delete
background+icon to error/onError so the fill and the glyph drawn on it
cannot drift apart.

The four category dots (deepOrange/amber/blueGrey/teal) stay literals,
with a comment saying why: they encode which category an item is, not a
theme role, and a single-accent system has no four distinct hues to lend
them. Mapping them onto primary/secondary/tertiary would make the
categories stop being distinguishable, which is their only job. They are
tracked in nielsen-audit.md as needing a categorical ramp. Note this
decouples the default dot from the old seed -- both were Colors.teal, so
they used to match by coincidence rather than by design.

Verified: 60/60 tests, 100% line coverage across 11 lib files, analyze
clean apart from four pre-existing unnecessary_unawaited infos. A
temporary assertion pumped the real app and read primary=ffb8862e and
surface=fff6f4f3 off a live Scaffold, confirming the palette changed
rather than merely compiling. Colour literals: 15 -> 4.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XS1m7b1piBtAL22USLW6jL

billsplit/lib/main.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 
 import 'package:billsplit/state/app_state.dart';
 import 'package:billsplit/ui/home_screen.dart';
+import 'package:design_system/design_system.dart';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
@@ -27,9 +28,10 @@ class BillSplitApp extends StatelessWidget {
       child: MaterialApp(
         title: 'BillSplit',
         debugShowCheckedModeBanner: false,
-        theme: ThemeData(
-          colorScheme: ColorScheme.fromSeed(seedColor: Colors.teal),
-        ),
+        // The shared light theme, replacing a teal-seeded scheme. Light, not
+        // dark: the receipt screens carry black-on-white helper text, so
+        // switching brightness here would make it unreadable.
+        theme: buildLightTheme(),
         home: const HomeScreen(),
       ),
     );

billsplit/lib/ui/assignment_sheet.dart

@@ -1,6 +1,7 @@
 import 'package:billsplit/domain/models.dart';
 import 'package:billsplit/domain/split_engine.dart';
 import 'package:billsplit/state/app_state.dart';
+import 'package:design_system/design_system.dart';
 import 'package:flutter/material.dart';
 
 /// Bottom sheet to assign an item: quick modes (everyone / drinkers /
@@ -126,8 +127,10 @@ Future<void> showAssignmentSheet(
                           'totals until someone is assigned.'
                       : _eachLabel(resolved.length, item.totalGr),
                   style: TextStyle(
-                    color: resolved.isEmpty ? Colors.red : Colors.black54,
-                    fontSize: 12,
+                    color: resolved.isEmpty
+                        ? Theme.of(c).colorScheme.error
+                        : Theme.of(c).colorScheme.onSurfaceVariant,
+                    fontSize: AppTextSize.caption,
                   ),
                 ),
               ],

billsplit/lib/ui/home_screen.dart

@@ -6,6 +6,7 @@ import 'package:billsplit/domain/models.dart';
 import 'package:billsplit/state/app_state.dart';
 import 'package:billsplit/ui/people_screen.dart';
 import 'package:billsplit/ui/receipt_screen.dart';
+import 'package:design_system/design_system.dart';
 import 'package:file_selector/file_selector.dart';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
@@ -45,11 +46,17 @@ class HomeScreen extends StatelessWidget {
                     return Dismissible(
                       key: ValueKey(r.id),
                       direction: DismissDirection.endToStart,
+                      // The delete affordance revealed by swiping. The fill
+                      // and the icon on top of it must move together, hence
+                      // error/onError rather than two independent colours.
                       background: Container(
-                        color: Colors.red.shade300,
+                        color: Theme.of(context).colorScheme.error,
                         alignment: Alignment.centerRight,
-                        padding: const EdgeInsets.only(right: 16),
-                        child: const Icon(Icons.delete, color: Colors.white),
+                        padding: const EdgeInsets.only(right: AppSpacing.md),
+                        child: Icon(
+                          Icons.delete,
+                          color: Theme.of(context).colorScheme.onError,
+                        ),
                       ),
                       confirmDismiss: (_) => _confirmDelete(context, r),
                       onDismissed: (_) => state.removeReceipt(r),

billsplit/lib/ui/item_edit.dart

@@ -1,5 +1,6 @@
 import 'package:billsplit/domain/models.dart';
 import 'package:billsplit/state/app_state.dart';
+import 'package:design_system/design_system.dart';
 import 'package:flutter/material.dart';
 
 /// Add ([item] == null) or edit an item. The user types amounts in złoty
@@ -65,10 +66,13 @@ Future<void> showItemEditDialog(
                     setState(() => category = v ?? Categories.general),
               ),
               const SizedBox(height: 4),
-              const Text(
+              Text(
                 'Alcohol items default to drinkers only; everything else to '
                 'everyone. Fine-tune by tapping the item afterwards.',
-                style: TextStyle(fontSize: 11, color: Colors.black54),
+                style: TextStyle(
+                  fontSize: AppTextSize.caption,
+                  color: Theme.of(c).colorScheme.onSurfaceVariant,
+                ),
               ),
             ],
           ),
@@ -80,7 +84,10 @@ Future<void> showItemEditDialog(
                 state.mutate(() => receipt.items.remove(item));
                 Navigator.pop(c);
               },
-              child: const Text('Delete', style: TextStyle(color: Colors.red)),
+              child: Text(
+                'Delete',
+                style: TextStyle(color: Theme.of(c).colorScheme.error),
+              ),
             ),
           TextButton(
             onPressed: () => Navigator.pop(c),

billsplit/lib/ui/people_screen.dart

@@ -1,5 +1,6 @@
 import 'package:billsplit/domain/models.dart';
 import 'package:billsplit/state/app_state.dart';
+import 'package:design_system/design_system.dart';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
@@ -25,12 +26,15 @@ class PeopleScreen extends StatelessWidget {
           for (final p in state.people) _PersonTile(person: p),
           const Divider(height: 32),
           const _SectionHeader('Groups'),
-          const Padding(
-            padding: EdgeInsets.symmetric(horizontal: 16),
+          Padding(
+            padding: const EdgeInsets.symmetric(horizontal: 16),
             child: Text(
               'Drinkers / non-drinkers are built in (the toggle on each '
               'person). Groups below are extra tags, e.g. "meat eaters".',
-              style: TextStyle(fontSize: 12, color: Colors.black54),
+              style: TextStyle(
+                fontSize: AppTextSize.caption,
+                color: Theme.of(context).colorScheme.onSurfaceVariant,
+              ),
             ),
           ),
           for (final g in state.groups) _GroupTile(group: g),

billsplit/lib/ui/receipt_screen.dart

@@ -234,7 +234,7 @@ class _ItemsList extends StatelessWidget {
           subtitle: Text(
             _assignmentLabel(item, people.length, state),
             style: TextStyle(
-              color: unassigned ? Colors.red : null,
+              color: unassigned ? Theme.of(context).colorScheme.error : null,
               fontWeight: unassigned ? FontWeight.bold : null,
             ),
           ),
@@ -272,6 +272,12 @@ class _CategoryDot extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final color = switch (category) {
+      // Categorical encoding, deliberately NOT mapped onto ColorScheme slots.
+      // These four say "which category", not "which theme role", and a
+      // single-accent design system has no four distinct hues to lend. Folding
+      // them into primary/secondary/tertiary would make the categories stop
+      // being distinguishable, which is the only job they have. Tracked in
+      // unified-design-system/nielsen-audit.md as needing a categorical ramp.
       Categories.alcohol => Colors.deepOrange,
       Categories.mixer => Colors.amber,
       Categories.deposit => Colors.blueGrey,
@@ -300,14 +306,16 @@ class _TotalsView extends StatelessWidget {
       children: [
         if (split.unassigned.isNotEmpty)
           Card(
-            color: Colors.red.shade50,
+            color: Theme.of(context).colorScheme.errorContainer,
             child: Padding(
               padding: const EdgeInsets.all(12),
               child: Text(
                 '${split.unassigned.length} item(s) have nobody assigned and '
                 'are excluded from totals:\n'
                 '${split.unassigned.map((i) => '· ${i.name}').join('\n')}',
-                style: const TextStyle(color: Colors.red),
+                style: TextStyle(
+                  color: Theme.of(context).colorScheme.onErrorContainer,
+                ),
               ),
             ),
           ),
@@ -347,7 +355,7 @@ class _TotalsView extends StatelessWidget {
   }) {
     final style = TextStyle(
       fontWeight: bold || warn ? FontWeight.bold : null,
-      color: warn ? Colors.red : null,
+      color: warn ? Theme.of(context).colorScheme.error : null,
     );
     return Padding(
       padding: const EdgeInsets.symmetric(vertical: 2),

billsplit/pubspec.lock

@@ -97,6 +97,15 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.7"
+  design_system:
+    dependency: "direct main"
+    description:
+      path: design_system
+      ref: "design_system-v0.1.0"
+      resolved-ref: e6c9af81e9ac3000d87d9168d3ef87ba2979258b
+      url: "https://github.com/kuhyx/utils"
+    source: git
+    version: "0.1.0"
   equatable:
     dependency: transitive
     description:
@@ -332,10 +341,10 @@ packages:
     dependency: transitive
     description:
       name: matcher
-      sha256: dc0b7dc7651697ea4ff3e69ef44b0407ea32c487a39fff6a4004fa585e901861
+      sha256: "31bd099b47c10cd1aeb55146a2d46ce0277630ecef3f7dae54ad7873f36696cd"
       url: "https://pub.dev"
     source: hosted
-    version: "0.12.19"
+    version: "0.12.20"
   material_color_utilities:
     dependency: transitive
     description:
@@ -348,10 +357,10 @@ packages:
     dependency: transitive
     description:
       name: meta
-      sha256: "1741988757a65eb6b36abe716829688cf01910bbf91c34354ff7ec1c3de2b349"
+      sha256: "307249ce4ff29d58a18e97f6345f539382eb9c9c29ecda628900f31de0443dd9"
       url: "https://pub.dev"
     source: hosted
-    version: "1.18.0"
+    version: "1.19.0"
   nested:
     dependency: transitive
     description:
@@ -529,10 +538,10 @@ packages:
     dependency: transitive
     description:
       name: test_api
-      sha256: "949a932224383300f01be9221c39180316445ecb8e7547f70a41a35bf421fb9e"
+      sha256: "2a122cbe059f8b610d3a5415f42e255b6c17b1f21eee1d960f31080237fb4f11"
       url: "https://pub.dev"
     source: hosted
-    version: "0.7.11"
+    version: "0.7.12"
   typed_data:
     dependency: transitive
     description:
@@ -545,10 +554,10 @@ packages:
     dependency: transitive
     description:
       name: vector_math
-      sha256: d530bd74fea330e6e364cda7a85019c434070188383e1cd8d9777ee586914c5b
+      sha256: f36f9f3be64c6198714492bb455c11056e33e2f85d9a0b676a48301e44fdcf47
       url: "https://pub.dev"
     source: hosted
-    version: "2.2.0"
+    version: "2.4.2"
   very_good_analysis:
     dependency: "direct dev"
     description:
@@ -598,5 +607,5 @@ packages:
     source: hosted
     version: "3.1.3"
 sdks:
-  dart: ">=3.12.0 <4.0.0"
+  dart: ">=3.12.2 <4.0.0"
   flutter: ">=3.44.0"

billsplit/pubspec.yaml

@@ -7,6 +7,13 @@ environment:
   sdk: ^3.5.0
 
 dependencies:
+  # The shared palette and scales.
+  design_system:
+    git:
+      url: https://github.com/kuhyx/utils
+      ref: design_system-v0.1.0
+      path: design_system
+
   excel: ^4.0.6
   file_selector: ^1.1.0
   flutter: