Page loads of errors show an icon page

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/lib/desktop/error_page.dart

@@ -0,0 +1,34 @@
+/// A wordless page for an error the window itself loaded: Chrome would
+/// otherwise paint its own "This page isn't working" sentences over an
+/// empty error body. Only the title carries the app's name (OS-level label).
+library error_page;
+
+import 'dart:io';
+
+/// HTML page with only an icon — no words painted by the browser.
+const String errorIconPage =
+    '<!doctype html><html><head><meta charset="utf-8"> '
+    '<title>Diet Guard</title><style>html,body{margin:0;height:100%;'
+    'background:#211D1B;display:flex;align-items:center;'
+    'justify-content:center}</style></head><body> '
+    '<svg width="96" height="96" viewBox="0 0 24 24" fill="none" '
+    ' stroke="#A39892" stroke-width="2" stroke-linecap="round" '
+    ' aria-hidden="true"><circle cx="12" cy="12" r="9"/>'
+    '<line x1="5.6" y1="18.4" x2="18.4" y2="5.6"/></svg></body></html>';
+
+/// Whether [request] is the window loading a page, not the app's fetch().
+bool isNavigation(HttpRequest request) =>
+    request.headers.value('sec-fetch-mode') == 'navigate';
+
+/// Replaces an error answer to a page load with [errorIconPage]. Does
+/// nothing for fetch() calls, successes, or a response already written.
+void paintErrorIcon(HttpRequest request) {
+  final response = request.response;
+  if (!isNavigation(request) || response.statusCode < 400) return;
+  try {
+    response.headers.contentType = ContentType.html;
+    response.write(errorIconPage);
+  } on Exception {
+    return; // body already sent: leave it alone
+  }
+}

app/lib/desktop/github_proxy.dart

@@ -19,6 +19,7 @@ library;
 import 'dart:convert';
 import 'dart:io';
 
+import 'package:diet_guard_app/desktop/error_page.dart' show isNavigation;
 import 'package:http/http.dart' as http;
 
 /// GitHub's REST API base, which [GitHubProxy] fronts.
@@ -177,9 +178,8 @@ class GitHubProxy {
   }
 
   void _passThrough(HttpRequest request, http.Response response) {
-    final isNavigate = request.headers.value('sec-fetch-mode') == 'navigate';
     request.response.statusCode = response.statusCode;
-    if (isNavigate) {
+    if (isNavigation(request)) {
       // Top-level navigation (window load) must not paint error JSON; the app's
       // own fetch() calls never send 'navigate', so they still get the body.
       return;

app/lib/desktop/wrapper_server.dart

@@ -24,6 +24,7 @@ import 'dart:io';
 import 'package:crdt_sync/crdt_sync.dart';
 
 import 'package:diet_guard_app/desktop/content_types.dart' as content_types;
+import 'package:diet_guard_app/desktop/error_page.dart';
 import 'package:diet_guard_app/desktop/github_proxy.dart';
 import 'package:diet_guard_app/services/desktop_wrapper.dart';
 import 'package:path/path.dart' as p;
@@ -91,6 +92,7 @@ class WrapperServer {
       } on Exception {
         request.response.statusCode = HttpStatus.internalServerError;
       }
+      paintErrorIcon(request);
       await request.response.close();
     }
   }

app/pubspec.yaml

@@ -2,7 +2,7 @@ name: diet_guard_app
 description: "Companion phone app for diet_guard: log meals on the go."
 publish_to: 'none'
 
-version: 1.0.152+1
+version: 1.0.153+1
 
 environment:
   sdk: ^3.12.2

app/test/desktop/desktop_wrapper_no_text_test.dart

@@ -13,6 +13,16 @@ import 'package:http/http.dart' as http;
 import 'package:http/testing.dart';
 import 'package:path/path.dart' as p;
 
+/// Removes <title> and <style> tags, strips all remaining HTML tags, and
+/// returns every [A-Za-z]{2,} match — i.e. all visible words the browser would
+/// paint for a top-level navigation.
+List<String> paintedWords(String html) {
+  var noTitle = html.replaceAll(RegExp(r'<title>.*?</title>', dotAll: true), '');
+  var noStyle = noTitle.replaceAll(RegExp(r'<style>.*?</style>', dotAll: true), '');
+  var noTags = noStyle.replaceAll(RegExp(r'<[^>]*>'), '');
+  return RegExp(r'[A-Za-z]{2,}').allMatches(noTags).map((m) => m.group(0)!).toList();
+}
+
 void main() {
   late Directory tempDir;
   late WrapperServer server;
@@ -69,6 +79,18 @@ void main() {
       expect(response.statusCode, status);
       expect(response.body, isEmpty, reason: 'nothing painted in the window');
     });
+
+    test('$method $path loaded as a page shows only an icon', () async {
+      final request = http.Request(method, url(path))
+        ..headers['sec-fetch-mode'] = 'navigate';
+      final response = await http.Response.fromStream(
+        await http.Client().send(request),
+      );
+      expect(response.statusCode, status);
+      expect(response.headers['content-type'], contains('text/html'));
+      expect(response.body, contains('<svg'));
+      expect(paintedWords(response.body), isEmpty);
+    });
   }
 
   group('proxy error responses', () {
@@ -105,14 +127,16 @@ void main() {
 
     Uri url(String path) => Uri.parse('http://localhost:${server.port}$path');
 
-    test('a proxy error loaded as a page has an empty body', () async {
+    test('a proxy error loaded as a page shows only an icon', () async {
       final request = http.Request('GET', url('/github/api/repos/o/r/contents/x'))
         ..headers['sec-fetch-mode'] = 'navigate';
       final response = await http.Response.fromStream(
         await http.Client().send(request),
       );
       expect(response.statusCode, HttpStatus.unauthorized);
-      expect(response.body, isEmpty, reason: 'nothing painted in the window');
+      expect(response.headers['content-type'], contains('text/html'));
+      expect(response.body, contains('<svg'));
+      expect(paintedWords(response.body), isEmpty);
     });
 
     test('the app fetch still gets the proxy error body', () async {