feat(flutter/theme): buildDarkTheme + buildLightTheme

Both builders construct a ThemeData with the matching brightness +
ColorScheme.dark/.light + the corresponding FabledSwordTheme
extension. buildThemeData() stays as a back-compat alias returning
the dark theme so the existing widget-test suite keeps using it
without churn.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/theme/theme_data.dart

@@ -4,8 +4,8 @@ import 'package:google_fonts/google_fonts.dart';
 import 'theme_extension.dart';
 import 'tokens.dart';
 
-ThemeData buildThemeData() {
-  final fs = FabledSwordTheme.fromTokens();
+ThemeData buildDarkTheme() {
+  final fs = FabledSwordTheme.dark();
   final colorScheme = ColorScheme.dark(
     surface: fs.iron,
     onSurface: fs.parchment,
@@ -14,16 +14,45 @@ ThemeData buildThemeData() {
     secondary: fs.moss,
     error: fs.error,
   );
-
   return ThemeData(
     useMaterial3: true,
+    brightness: Brightness.dark,
     colorScheme: colorScheme,
     scaffoldBackgroundColor: fs.obsidian,
     textTheme: GoogleFonts.interTextTheme().apply(
       bodyColor: fs.parchment,
       displayColor: fs.parchment,
     ),
-    fontFamily: FabledSwordTokens.fontBody,
+    fontFamily: FabledSwordFlatTokens.fontBody,
     extensions: <ThemeExtension<dynamic>>[fs],
   );
 }
+
+ThemeData buildLightTheme() {
+  final fs = FabledSwordTheme.light();
+  final colorScheme = ColorScheme.light(
+    surface: fs.iron,
+    onSurface: fs.parchment, // semantic — light's "parchment" is dark text
+    primary: fs.accent,
+    onPrimary: FabledSwordFlatTokens.onAction,
+    secondary: fs.moss,
+    error: fs.error,
+  );
+  return ThemeData(
+    useMaterial3: true,
+    brightness: Brightness.light,
+    colorScheme: colorScheme,
+    scaffoldBackgroundColor: fs.obsidian, // semantic — light's obsidian is the bg
+    textTheme: GoogleFonts.interTextTheme().apply(
+      bodyColor: fs.parchment,
+      displayColor: fs.parchment,
+    ),
+    fontFamily: FabledSwordFlatTokens.fontBody,
+    extensions: <ThemeExtension<dynamic>>[fs],
+  );
+}
+
+/// Back-compat alias. Existing tests + code call buildThemeData(); it
+/// returns the dark theme to preserve current behaviour. New code
+/// should prefer buildDarkTheme()/buildLightTheme() explicitly.
+ThemeData buildThemeData() => buildDarkTheme();