feat: extend SeasonTheme with visual identity fields (ARGB ints)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/game/models/season.dart

@@ -1,18 +1,62 @@
 import 'stage.dart';
 
+/// Visual identity of a season. Colors are int ARGB so this file stays
+/// pure Dart (architecture guard forbids Flutter imports here).
 class SeasonTheme {
-  const SeasonTheme({required this.tileSet, required this.background});
+  const SeasonTheme({
+    this.tileSet = 'spring',
+    this.background = '',
+    this.backgroundGradient = defaultGradient,
+    this.accentColor = 0xFFFF7EB3,
+    this.particleType = 'petals',
+    this.tilePalette,
+    this.boardTint,
+  });
 
   factory SeasonTheme.fromJson(Map<String, dynamic> json) => SeasonTheme(
-        tileSet: json['tileSet'] as String,
-        background: json['background'] as String,
+        tileSet: (json['tileSet'] as String?) ?? 'spring',
+        background: (json['background'] as String?) ?? '',
+        backgroundGradient: json['backgroundGradient'] != null
+            ? [for (final c in json['backgroundGradient'] as List) c as int]
+            : defaultGradient,
+        accentColor: (json['accentColor'] as int?) ?? 0xFFFF7EB3,
+        particleType: (json['particleType'] as String?) ?? 'petals',
+        tilePalette: json['tilePalette'] != null
+            ? [for (final c in json['tilePalette'] as List) c as int]
+            : null,
+        boardTint: json['boardTint'] as int?,
       );
 
+  /// Season 1 "First Bloom": deep navy dusk.
+  static const defaultGradient = [0xFF0E1430, 0xFF16204A, 0xFF2A2E5E];
+
+  static const fallback = SeasonTheme();
+
   final String tileSet;
   final String background;
 
-  Map<String, dynamic> toJson() =>
-      {'tileSet': tileSet, 'background': background};
+  /// Top-to-bottom screen gradient, int ARGB.
+  final List<int> backgroundGradient;
+  final int accentColor;
+
+  /// petals | snow | leaves | none
+  final String particleType;
+
+  /// Optional 8-color tile override; null = built-in candy palette.
+  final List<int>? tilePalette;
+
+  /// Optional board background override.
+  final int? boardTint;
+
+  Map<String, dynamic> toJson() => {
+        'tileSet': tileSet,
+        'background': background,
+        'backgroundGradient': backgroundGradient,
+        'accentColor': accentColor,
+        'particleType': particleType,
+        if (tilePalette != null) 'tilePalette': tilePalette,
+        if (boardTint != null) 'boardTint': boardTint,
+      };
 }
 
 /// A season's full content: metadata, theme, and its stages. The unit of