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superseded by this Flutter web build).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/screens/chat_screen.dart

@@ -0,0 +1,159 @@
+import 'package:flutter/material.dart';
+
+import '../api.dart';
+import '../main.dart';
+
+/// Read-only chat view. Sending a message is a WebSocket-only path on the
+/// backend (`chat:send`); wiring that up is a follow-up.
+class ChatScreen extends StatefulWidget {
+  const ChatScreen({super.key, required this.kcId});
+  final String kcId;
+
+  @override
+  State<ChatScreen> createState() => _ChatScreenState();
+}
+
+class _ChatScreenState extends State<ChatScreen> {
+  Future<List<ChatChannel>>? _future;
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    _future ??= AppScope.of(context).api.channels(widget.kcId);
+  }
+
+  static const _typeLabels = {
+    'GEMEINDE_GRUPPE': 'Gemeinde-Gruppe',
+    'DIREKT': 'Direktnachricht',
+    'LT_UEBERGREIFEND': 'Leitungsteam',
+    'BROADCAST': 'Ankündigungen',
+  };
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('Chat')),
+      body: FutureBuilder<List<ChatChannel>>(
+        future: _future,
+        builder: (context, snap) {
+          if (snap.connectionState != ConnectionState.done) {
+            return const Center(child: CircularProgressIndicator());
+          }
+          if (snap.hasError) {
+            return Center(
+              child: Padding(
+                padding: const EdgeInsets.all(24),
+                child: Text('${snap.error}', textAlign: TextAlign.center),
+              ),
+            );
+          }
+          final channels = snap.data!;
+          if (channels.isEmpty) {
+            return const Center(child: Text('Keine Kanäle sichtbar.'));
+          }
+          return ListView(
+            children: [
+              for (final c in channels)
+                ListTile(
+                  leading: const Icon(Icons.tag),
+                  title: Text(_typeLabels[c.type] ?? c.type),
+                  subtitle: Text(c.id),
+                  onTap: () => Navigator.of(context).push(
+                    MaterialPageRoute(
+                      builder: (_) => _ChannelMessages(
+                        channelId: c.id,
+                        title: _typeLabels[c.type] ?? c.type,
+                      ),
+                    ),
+                  ),
+                ),
+            ],
+          );
+        },
+      ),
+    );
+  }
+}
+
+class _ChannelMessages extends StatefulWidget {
+  const _ChannelMessages({required this.channelId, required this.title});
+  final String channelId;
+  final String title;
+
+  @override
+  State<_ChannelMessages> createState() => _ChannelMessagesState();
+}
+
+class _ChannelMessagesState extends State<_ChannelMessages> {
+  Future<List<ChatMessage>>? _future;
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    _future ??= AppScope.of(context).api.messages(widget.channelId);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: Text(widget.title)),
+      body: FutureBuilder<List<ChatMessage>>(
+        future: _future,
+        builder: (context, snap) {
+          if (snap.connectionState != ConnectionState.done) {
+            return const Center(child: CircularProgressIndicator());
+          }
+          if (snap.hasError) {
+            return Center(
+              child: Padding(
+                padding: const EdgeInsets.all(24),
+                child: Text('${snap.error}', textAlign: TextAlign.center),
+              ),
+            );
+          }
+          final messages = snap.data!;
+          if (messages.isEmpty) {
+            return const Center(child: Text('Noch keine Nachrichten.'));
+          }
+          return ListView.builder(
+            padding: const EdgeInsets.all(12),
+            itemCount: messages.length,
+            itemBuilder: (context, i) {
+              final m = messages[i];
+              return Align(
+                alignment: Alignment.centerLeft,
+                child: Container(
+                  margin: const EdgeInsets.only(bottom: 8),
+                  padding: const EdgeInsets.all(10),
+                  decoration: BoxDecoration(
+                    color: Theme.of(context).colorScheme.surfaceContainerHighest,
+                    borderRadius: BorderRadius.circular(10),
+                  ),
+                  child: Column(
+                    crossAxisAlignment: CrossAxisAlignment.start,
+                    children: [
+                      Text(m.body),
+                      const SizedBox(height: 2),
+                      Text(
+                        m.createdAt,
+                        style: Theme.of(context).textTheme.labelSmall,
+                      ),
+                    ],
+                  ),
+                ),
+              );
+            },
+          );
+        },
+      ),
+      bottomNavigationBar: const Padding(
+        padding: EdgeInsets.all(12),
+        child: Text(
+          'Senden folgt (WebSocket) — aktuell nur Lesen.',
+          textAlign: TextAlign.center,
+          style: TextStyle(fontSize: 12),
+        ),
+      ),
+    );
+  }
+}