feat(client): Flutter app (web target) — Phase 7 start

Single Flutter codebase under client/app/ with web enabled (mobile/desktop
can be added later; lib/ is platform-agnostic). Talks to the NestJS backend
via a thin REST wrapper; API_BASE is a --dart-define (defaults to the local
backend).

Screens:
- Login: Konfi/guest (invite code), local Teamer password login, Teamer
  invite redemption. Token persisted in shared_preferences, restored on
  start; GET /auth/me drives a role-aware home.
- Workshop-Wahl (guests): loads /wahl/guest/overview, ordered pick of up to
  3 workshops, submits to /wahl/:id/teilnehmer.
- Dateien: /files/:kcId list.
- Chat: channel + message list (read-only; WS send is a follow-up).

State: AppState (ChangeNotifier) exposed via an InheritedNotifier
(AppScope) — no third-party state package. flutter analyze clean,
flutter build web --release passes, one widget smoke test.

Also: interim client/web/ HTML placeholder stays as-is (per plan it is
superseded by this Flutter web build).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

client/app/lib/main.dart

@@ -0,0 +1,58 @@
+import 'package:flutter/material.dart';
+import 'package:http/http.dart' as http;
+
+import 'api.dart';
+import 'screens/home_screen.dart';
+import 'screens/login_screen.dart';
+
+void main() {
+  final state = AppState(Api(http.Client()))..bootstrap();
+  runApp(KcApp(state: state));
+}
+
+/// Minimal InheritedNotifier so screens can read `AppState.of(context)` and
+/// rebuild on change — no third-party state management.
+class AppScope extends InheritedNotifier<AppState> {
+  const AppScope({super.key, required AppState state, required super.child})
+      : super(notifier: state);
+
+  static AppState of(BuildContext context) {
+    final scope = context.dependOnInheritedWidgetOfExactType<AppScope>();
+    assert(scope != null, 'AppScope missing above this widget');
+    return scope!.notifier!;
+  }
+}
+
+class KcApp extends StatelessWidget {
+  const KcApp({super.key, required this.state});
+  final AppState state;
+
+  @override
+  Widget build(BuildContext context) {
+    return AppScope(
+      state: state,
+      child: MaterialApp(
+        title: 'KC-App',
+        debugShowCheckedModeBanner: false,
+        theme: ThemeData(
+          colorSchemeSeed: const Color(0xFF3B5BA5),
+          useMaterial3: true,
+        ),
+        home: const _AuthGate(),
+      ),
+    );
+  }
+}
+
+class _AuthGate extends StatelessWidget {
+  const _AuthGate();
+
+  @override
+  Widget build(BuildContext context) {
+    final state = AppScope.of(context);
+    if (state.loading) {
+      return const Scaffold(body: Center(child: CircularProgressIndicator()));
+    }
+    return state.isLoggedIn ? const HomeScreen() : const LoginScreen();
+  }
+}