feat: glossy tile rendering and per-season theme colors

Introduces candy-gloss tile rendering (diagonal gradient + glass highlight
+ optional glow) via a shared paintGlossyTile() in tile_painter.dart,
applied to board filled-cells and tray/drag-overlay pieces. Adds
ThemeColors to palette.dart for UI-layer season color resolution, and
activeThemeProvider for one-call access to the active season's theme.
Regenerates the game_screen golden to reflect the new look.

lib/state/providers.dart

@@ -45,3 +45,10 @@ final seasonsProvider = FutureProvider<List<SeasonPack>>(
 final streakProvider = NotifierProvider<StreakNotifier, StreakState>(
   StreakNotifier.new,
 );
+
+/// The visual theme of whatever season is in play; fallback outside seasons
+/// (home, endless). Pure model — UI converts via ThemeColors.
+final activeThemeProvider = Provider<SeasonTheme>((ref) {
+  final flow = ref.watch(seasonFlowProvider);
+  return flow?.pack.theme ?? SeasonTheme.fallback;
+});