chore: scaffold the Serverpod 4 build

Deviations from a stock `serverpod create`:
- Dropped Flutter web support, for two reasons:
  - Appflowy editor had issues in the past, potentially related to the
    Flutter-Web IME bridge. Upstream has also open bugs in relation to
that:
    - AppFlowy-IO/appflowy-editor#1029 [Bug][Web] Unwanted space on click
       and overall space bugs (open)
    - AppFlowy-IO/appflowy-editor#782 [Bug] Editor auto-scrolling when
       pressing space too quickly if placed inside a scrollable (open)
  - Offline-first is a required feature. serverpod_offline_sync's SQLite
    support in browsers, maybe via sqlite3.wasm + OPFS, is not
documented.
- SDK constraints raised to ^3.13.0, instead of ^3.12.2, so enjoy the
  newest language features.
- Pinned to Flutter 3.47.4 via FVM.

Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/main.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/material.dart';
+
+import 'client.dart';
+import 'screens/greetings_screen.dart';
+
+void main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  await initializeClient();
+  runApp(const MyApp());
+}
+
+/// Builds a theme for the given [brightness].
+ThemeData _buildTheme(Brightness brightness) {
+  return ThemeData(
+    colorScheme: ColorScheme.fromSeed(
+      seedColor: Colors.blue,
+      brightness: brightness,
+    ),
+  );
+}
+
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Serverpod Demo',
+      theme: _buildTheme(Brightness.light),
+      darkTheme: _buildTheme(Brightness.dark),
+      themeMode: ThemeMode.system,
+      home: const MyHomePage(title: 'Serverpod Example'),
+    );
+  }
+}
+
+class MyHomePage extends StatelessWidget {
+  const MyHomePage({super.key, required this.title});
+
+  final String title;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: Text(title)),
+      body: const GreetingsScreen(),
+      // To test authentication in this example app, uncomment the line below
+      // and comment out the line above. This wraps the GreetingsScreen with a
+      // SignInScreen, which automatically shows a sign-in UI when the user is
+      // not authenticated and displays the GreetingsScreen once they sign in.
+      //
+      // body: SignInScreen(
+      //   child: GreetingsScreen(
+      //     onSignOut: () async {
+      //       await client.auth.signOutDevice();
+      //     },
+      //   ),
+      // ),
+    );
+  }
+}