Phase 4 Stage 0: design system foundation (client_app)

- HaloTokens, HaloSpacing, HaloRadius, HaloMotion, HaloShadows (warm palette;
  calm/playful stubbed for phase 5).
- Bundled Bricolage Grotesque, Poppins, JetBrains Mono (~1.2 MB total, OFL).
- haloThemeData() wired into MaterialApp.router with Figma-aligned text
  scale, pill ElevatedButton, 64px input height, 24px-corner BottomSheet,
  dark pill SnackBar.
- Halo* widget primitives: Button, Orb, StepDots, BottomSheet, Popup,
  Snackbar, Chip.
- Dev-only /_theme_preview route gated by --dart-define=THEME_PREVIEW=true
  for visual reference during stages 2-8.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client_app/lib/main.dart

@@ -7,6 +7,7 @@ import 'core/auth/auth_notifier.dart';
 import 'core/chat/active_session_notifier.dart';
 import 'core/chat/chat_notifier.dart';
 import 'core/notifications/notification_service.dart';
+import 'core/theme/halo_theme.dart';
 import 'firebase_options.dart';
 import 'router.dart';
 
@@ -83,6 +84,7 @@ class _AppState extends ConsumerState<App> {
 
     return MaterialApp.router(
       title: 'Halo Bestie',
+      theme: haloThemeData(),
       routerConfig: router,
     );
   }