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/ui/widgets/piece_painter.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 
 import '../../game/models/piece.dart';
 import '../theme/palette.dart';
+import 'tile_painter.dart';
 
 /// Draws a piece as rounded tiles at a given cell size; reused by the tray,
 /// the drag overlay, and ghost previews.
@@ -12,8 +13,6 @@ void paintPiece(
   Offset origin = Offset.zero,
   Color? overrideColor,
 }) {
-  final paint = Paint()
-    ..color = overrideColor ?? GamePalette.tile(piece.colorId);
   final inset = cellSize * 0.05;
   final radius = Radius.circular(cellSize * 0.18);
   for (final (dx, dy) in piece.offsets) {
@@ -23,17 +22,13 @@ void paintPiece(
       cellSize - inset * 2,
       cellSize - inset * 2,
     );
-    canvas.drawRRect(RRect.fromRectAndRadius(rect, radius), paint);
-    if (overrideColor == null) {
-      // Subtle top highlight for depth.
-      final highlight = Paint()..color = Colors.white.withValues(alpha: 0.18);
+    if (overrideColor != null) {
       canvas.drawRRect(
-        RRect.fromRectAndRadius(
-          Rect.fromLTWH(rect.left, rect.top, rect.width, rect.height * 0.32),
-          radius,
-        ),
-        highlight,
+        RRect.fromRectAndRadius(rect, radius),
+        Paint()..color = overrideColor,
       );
+    } else {
+      paintGlossyTile(canvas, rect, GamePalette.tile(piece.colorId));
     }
   }
 }