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')),
       ),
     );

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],
+  );
+}

flutter_client/lib/theme/theme_extension.dart

@@ -0,0 +1,68 @@
+import 'package:flutter/material.dart';
+
+import 'tokens.dart';
+
+class FabledSwordTheme extends ThemeExtension<FabledSwordTheme> {
+  const FabledSwordTheme({
+    required this.accent,
+    required this.obsidian,
+    required this.iron,
+    required this.slate,
+    required this.pewter,
+    required this.parchment,
+    required this.vellum,
+    required this.ash,
+    required this.moss,
+    required this.bronze,
+    required this.oxblood,
+    required this.warning,
+    required this.error,
+    required this.info,
+    required this.display,
+    required this.body,
+    required this.mono,
+  });
+
+  final Color accent;
+  final Color obsidian, iron, slate, pewter;
+  final Color parchment, vellum, ash;
+  final Color moss, bronze, oxblood;
+  final Color warning, error, info;
+  final TextStyle display, body, mono;
+
+  static FabledSwordTheme fromTokens() => FabledSwordTheme(
+        accent: FabledSwordTokens.accent,
+        obsidian: FabledSwordTokens.obsidian,
+        iron: FabledSwordTokens.iron,
+        slate: FabledSwordTokens.slate,
+        pewter: FabledSwordTokens.pewter,
+        parchment: FabledSwordTokens.parchment,
+        vellum: FabledSwordTokens.vellum,
+        ash: FabledSwordTokens.ash,
+        moss: FabledSwordTokens.moss,
+        bronze: FabledSwordTokens.bronze,
+        oxblood: FabledSwordTokens.oxblood,
+        warning: FabledSwordTokens.warning,
+        error: FabledSwordTokens.error,
+        info: FabledSwordTokens.info,
+        display: const TextStyle(fontFamily: FabledSwordTokens.fontDisplay),
+        body: const TextStyle(fontFamily: FabledSwordTokens.fontBody),
+        mono: const TextStyle(fontFamily: FabledSwordTokens.fontMono),
+      );
+
+  @override
+  FabledSwordTheme copyWith({
+    Color? accent,
+  }) =>
+      FabledSwordTheme(
+        accent: accent ?? this.accent,
+        obsidian: obsidian, iron: iron, slate: slate, pewter: pewter,
+        parchment: parchment, vellum: vellum, ash: ash,
+        moss: moss, bronze: bronze, oxblood: oxblood,
+        warning: warning, error: error, info: info,
+        display: display, body: body, mono: mono,
+      );
+
+  @override
+  FabledSwordTheme lerp(ThemeExtension<FabledSwordTheme>? other, double t) => this;
+}

flutter_client/test/theme/theme_extension_test.dart

@@ -0,0 +1,26 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:minstrel/theme/theme_data.dart';
+import 'package:minstrel/theme/theme_extension.dart';
+import 'package:minstrel/theme/tokens.dart';
+
+void main() {
+  testWidgets('Theme exposes FabledSwordTheme with expected accent', (tester) async {
+    late FabledSwordTheme captured;
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: buildThemeData(),
+        home: Builder(
+          builder: (ctx) {
+            captured = Theme.of(ctx).extension<FabledSwordTheme>()!;
+            return const SizedBox.shrink();
+          },
+        ),
+      ),
+    );
+    expect(captured.accent, FabledSwordTokens.accent);
+    expect(captured.parchment, FabledSwordTokens.parchment);
+    expect(captured.body.fontFamily, FabledSwordTokens.fontBody);
+  });
+}