Initial commit: watch-together phases 0-4 + presence hardening, internet-tested

apps/desktop/lib/app.dart

@@ -0,0 +1,49 @@
+import 'dart:io';
+
+import 'package:flutter/material.dart';
+import 'package:signals/signals_flutter.dart';
+
+import 'src/screens/connect_screen.dart';
+import 'src/screens/legacy_session_screen.dart';
+import 'src/screens/minimal_session_screen.dart';
+import 'src/screens/nosheet_session_screen.dart';
+import 'src/screens/session_screen.dart';
+import 'src/screens/welcome_screen.dart';
+import 'src/state/session_state.dart';
+
+bool get useLegacySession =>
+    Platform.environment['SYNC_RELAY_UI'] == 'legacy';
+
+bool get useMinimalSession =>
+    Platform.environment['SYNC_RELAY_UI'] == 'minimal';
+
+bool get useNosheetSession =>
+    Platform.environment['SYNC_RELAY_UI'] == 'nosheet';
+
+class WatchTogetherApp extends StatelessWidget {
+  const WatchTogetherApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final session = SessionState.instance;
+    return MaterialApp(
+      title: 'Watch Together',
+      theme: ThemeData.dark(),
+      home: SignalBuilder(
+        builder: (context) {
+          return switch (session.route.value) {
+            AppRoute.welcome => const WelcomeScreen(),
+            AppRoute.connect => const ConnectScreen(),
+            AppRoute.session => useMinimalSession
+                ? const MinimalSessionScreen()
+                : useNosheetSession
+                    ? const NosheetSessionScreen()
+                    : useLegacySession
+                        ? const LegacySessionScreen()
+                        : const SessionScreen(),
+          };
+        },
+      ),
+    );
+  }
+}