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/theme/theme_data.dart

@@ -0,0 +1,29 @@
+import 'package:flutter/material.dart';
+import 'package:google_fonts/google_fonts.dart';
+
+import 'theme_extension.dart';
+import 'tokens.dart';
+
+ThemeData buildThemeData() {
+  final fs = FabledSwordTheme.fromTokens();
+  final colorScheme = ColorScheme.dark(
+    surface: fs.iron,
+    onSurface: fs.parchment,
+    primary: fs.accent,
+    onPrimary: fs.parchment,
+    secondary: fs.moss,
+    error: fs.error,
+  );
+
+  return ThemeData(
+    useMaterial3: true,
+    colorScheme: colorScheme,
+    scaffoldBackgroundColor: fs.obsidian,
+    textTheme: GoogleFonts.interTextTheme().apply(
+      bodyColor: fs.parchment,
+      displayColor: fs.parchment,
+    ),
+    fontFamily: FabledSwordTokens.fontBody,
+    extensions: <ThemeExtension<dynamic>>[fs],
+  );
+}