feat(about): add About page with device/app info and issue reporting (#106)

Shows version, platform, OS version, screen resolution, Dart version, and
processor count in a markdown table. Buttons let users copy the info to
clipboard or open a pre-filled Codeberg issue.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/ui/router.dart

@@ -2,6 +2,7 @@ import 'package:go_router/go_router.dart';
 
 import 'package:sharedinbox/core/models/sieve_script.dart';
 
+import 'package:sharedinbox/ui/screens/about_screen.dart';
 import 'package:sharedinbox/ui/screens/account_export_screen.dart';
 import 'package:sharedinbox/ui/screens/account_import_screen.dart';
 import 'package:sharedinbox/ui/screens/account_list_screen.dart';
@@ -47,6 +48,10 @@ final router = GoRouter(
               path: 'changelog',
               builder: (ctx, state) => const ChangeLogScreen(),
             ),
+            GoRoute(
+              path: 'about',
+              builder: (ctx, state) => const AboutScreen(),
+            ),
             GoRoute(
               path: ':accountId/edit',
               builder: (ctx, state) => EditAccountScreen(