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superseded by this Flutter web build).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/web/manifest.json

@@ -0,0 +1,35 @@
+{
+    "name": "kc_app",
+    "short_name": "kc_app",
+    "start_url": ".",
+    "display": "standalone",
+    "background_color": "#0175C2",
+    "theme_color": "#0175C2",
+    "description": "A new Flutter project.",
+    "orientation": "portrait-primary",
+    "prefer_related_applications": false,
+    "icons": [
+        {
+            "src": "icons/Icon-192.png",
+            "sizes": "192x192",
+            "type": "image/png"
+        },
+        {
+            "src": "icons/Icon-512.png",
+            "sizes": "512x512",
+            "type": "image/png"
+        },
+        {
+            "src": "icons/Icon-maskable-192.png",
+            "sizes": "192x192",
+            "type": "image/png",
+            "purpose": "maskable"
+        },
+        {
+            "src": "icons/Icon-maskable-512.png",
+            "sizes": "512x512",
+            "type": "image/png",
+            "purpose": "maskable"
+        }
+    ]
+}