feat(flutter): FabledSwordTheme ThemeExtension + Material 3 ThemeData

Widgets read tokens via Theme.of(ctx).extension<FabledSwordTheme>().
Material 3 ColorScheme bound to FabledSword tokens. Body text uses
GoogleFonts.interTextTheme() so the fonts work without bundling .ttf
files (offline-from-cold-launch is a separate question once #357 lands).

flutter_client/lib/app.dart

@@ -1,13 +1,16 @@
 import 'package:flutter/material.dart';
 
+import 'theme/theme_data.dart';
+
 class MinstrelApp extends StatelessWidget {
   const MinstrelApp({super.key});
 
   @override
   Widget build(BuildContext context) {
-    return const MaterialApp(
+    return MaterialApp(
       title: 'Minstrel',
-      home: Scaffold(
+      theme: buildThemeData(),
+      home: const Scaffold(
         body: Center(child: Text('Minstrel — scaffold')),
       ),
     );