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superseded by this Flutter web build).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/.metadata

@@ -0,0 +1,30 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: "e8113bf45620cbeb8aff64947ee4c93e16adb4cf"
+  channel: "stable"
+
+project_type: app
+
+# Tracks metadata for the flutter migrate command
+migration:
+  platforms:
+    - platform: root
+      create_revision: e8113bf45620cbeb8aff64947ee4c93e16adb4cf
+      base_revision: e8113bf45620cbeb8aff64947ee4c93e16adb4cf
+    - platform: web
+      create_revision: e8113bf45620cbeb8aff64947ee4c93e16adb4cf
+      base_revision: e8113bf45620cbeb8aff64947ee4c93e16adb4cf
+
+  # User provided section
+
+  # List of Local paths (relative to this file) that should be
+  # ignored by the migrate tool.
+  #
+  # Files that are not part of the templates will be ignored by default.
+  unmanaged_files:
+    - 'lib/main.dart'
+    - 'ios/Runner.xcodeproj/project.pbxproj'