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superseded by this Flutter web build).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/pubspec.yaml

@@ -0,0 +1,21 @@
+name: kc_app
+description: "KC-App client — multi-tenant event, election and communication platform for Konfi-Castle events."
+publish_to: 'none'
+version: 0.1.0+1
+
+environment:
+  sdk: ^3.13.3
+
+dependencies:
+  flutter:
+    sdk: flutter
+  http: ^1.2.2
+  shared_preferences: ^2.3.2
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  flutter_lints: ^6.0.0
+
+flutter:
+  uses-material-design: true