Make AppStatusColors lookup null-safe; fix tests that never had it

Theme.of(context).extension<AppStatusColors>()! crashed in any context
that doesn't build its theme from buildAppTheme() -- which is most widget
tests, since they wrap in a bare MaterialApp(home: ...) rather than the
real app theme. That crash was latent since AppStatusColors was
introduced but only became reachable once main.dart actually started
using buildAppTheme() (previous commit), which is when 27-47 tests
started failing.

Fixed at both call sites (slot_selector_row.dart, day_status_calendar.dart)
by falling back to AppStatusColors.dark instead of force-unwrapping --
production code should never trust every context to fully replicate the
real theme. Also fixed day_status_calendar_test.dart, which asserted
against stale hardcoded Colors.* values from before the theme rewrite;
it now wraps in the real buildAppTheme() and asserts against the actual
tokens, so the assertions mean something again.

Verified: flutter analyze clean, 436/436 tests pass (up from 389/47).

app/lib/widgets/day_status_calendar.dart

@@ -103,7 +103,12 @@ class DayStatusCalendar extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final scheme = Theme.of(context).colorScheme;
-    final statusColors = Theme.of(context).extension<AppStatusColors>()!;
+    // A `!` here would crash in any context that doesn't build its theme
+    // from buildAppTheme() -- most widget tests just wrap in a bare
+    // MaterialApp(home: ...). Falling back to the app's own dark instance
+    // matches production exactly when the extension is genuinely absent.
+    final statusColors =
+        Theme.of(context).extension<AppStatusColors>() ?? AppStatusColors.dark;
     final year = month.year;
     final m = month.month;
     final daysInMonth = DateTime(year, m + 1, 0).day;

app/lib/widgets/slot_selector_row.dart

@@ -36,7 +36,12 @@ class SlotSelectorRow extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final scheme = Theme.of(context).colorScheme;
-    final statusColors = Theme.of(context).extension<AppStatusColors>()!;
+    // A `!` here would crash in any context that doesn't build its theme
+    // from buildAppTheme() -- most widget tests just wrap in a bare
+    // MaterialApp(home: ...). Falling back to the app's own dark instance
+    // matches production exactly when the extension is genuinely absent.
+    final statusColors =
+        Theme.of(context).extension<AppStatusColors>() ?? AppStatusColors.dark;
     final elapsed = elapsedSlots(now).toSet();
     // Snack has no logged/due/upcoming status like the hour chips, so it
     // uses `accent` as its "color" instead of a status color -- but still

app/test/widgets/day_status_calendar_test.dart

@@ -1,9 +1,20 @@
 import 'package:diet_guard_app/models/day_status.dart';
+import 'package:diet_guard_app/ui/theme.dart';
 import 'package:diet_guard_app/widgets/day_status_calendar.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-Widget _wrap(Widget child) => MaterialApp(home: Scaffold(body: child));
+// The real app theme, not a bare default MaterialApp -- the widget under
+// test reads its colors from Theme.of(context), so asserting against the
+// same theme the app actually ships is what makes these assertions mean
+// anything (a bare MaterialApp's own default colors would be arbitrary).
+final _theme = buildAppTheme();
+final _statusColors = _theme.extension<AppStatusColors>()!;
+
+Widget _wrap(Widget child) => MaterialApp(
+  theme: _theme,
+  home: Scaffold(body: child),
+);
 
 Color _colorOfDay(WidgetTester tester, String day) {
   final container = tester.widget<Container>(
@@ -95,7 +106,7 @@ void main() {
           ),
         ),
       );
-      expect(_colorOfDay(tester, '15'), Colors.green.shade700);
+      expect(_colorOfDay(tester, '15'), _statusColors.success);
     });
 
     testWidgets('colors a yellow day', (tester) async {
@@ -110,7 +121,7 @@ void main() {
           ),
         ),
       );
-      expect(_colorOfDay(tester, '15'), Colors.amber.shade700);
+      expect(_colorOfDay(tester, '15'), _statusColors.warning);
     });
 
     testWidgets('colors a red day', (tester) async {
@@ -125,7 +136,7 @@ void main() {
           ),
         ),
       );
-      expect(_colorOfDay(tester, '15'), Colors.red.shade400);
+      expect(_colorOfDay(tester, '15'), _theme.colorScheme.error);
     });
 
     testWidgets(
@@ -142,7 +153,7 @@ void main() {
             ),
           ),
         );
-        expect(_colorOfDay(tester, '15'), Colors.black);
+        expect(_colorOfDay(tester, '15'), _theme.colorScheme.surface);
       },
     );
 
@@ -161,7 +172,10 @@ void main() {
           ),
         );
         // Day 25 is after the reference "today" of June 10.
-        expect(_colorOfDay(tester, '25'), Colors.grey.shade800);
+        expect(
+          _colorOfDay(tester, '25'),
+          _theme.colorScheme.surfaceContainerHigh,
+        );
       },
     );
 
@@ -179,7 +193,7 @@ void main() {
           ),
         ),
       );
-      expect(_colorOfDay(tester, '20'), Colors.red.shade400);
+      expect(_colorOfDay(tester, '20'), _theme.colorScheme.error);
     });
 
     testWidgets('renders a month starting on Sunday correctly', (