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/board_painter.dart

@@ -6,6 +6,7 @@ import '../../game/models/piece.dart';
 import '../theme/palette.dart';
 import 'board_geometry.dart';
 import 'piece_painter.dart';
+import 'tile_painter.dart';
 
 /// Drag ghost preview: a piece hovering at a snapped anchor.
 class GhostSpec {
@@ -55,27 +56,20 @@ class BoardPainter extends CustomPainter {
       for (var x = 0; x < GridState.size; x++) {
         final rect = geo.cellRect(x, y).deflate(inset);
         final cell = grid.cellAt(x, y);
-        final paint = Paint()
-          ..color = switch (cell.type) {
-            CellType.empty => GamePalette.emptyCell,
-            CellType.filled => GamePalette.tile(cell.colorId),
-            CellType.gem => GamePalette.emptyCell,
-          };
-        canvas.drawRRect(RRect.fromRectAndRadius(rect, radius), paint);
-
-        if (cell.type == CellType.gem) {
-          _paintGem(canvas, rect);
-        } else if (cell.type == CellType.filled) {
-          final highlight = Paint()
-            ..color = Colors.white.withValues(alpha: 0.15);
-          canvas.drawRRect(
-            RRect.fromRectAndRadius(
-              Rect.fromLTWH(
-                  rect.left, rect.top, rect.width, rect.height * 0.32),
-              radius,
-            ),
-            highlight,
-          );
+        switch (cell.type) {
+          case CellType.empty:
+            canvas.drawRRect(
+              RRect.fromRectAndRadius(rect, radius),
+              Paint()..color = GamePalette.emptyCell,
+            );
+          case CellType.filled:
+            paintGlossyTile(canvas, rect, GamePalette.tile(cell.colorId));
+          case CellType.gem:
+            canvas.drawRRect(
+              RRect.fromRectAndRadius(rect, radius),
+              Paint()..color = GamePalette.emptyCell,
+            );
+            _paintGem(canvas, rect);
         }
       }
     }
@@ -111,6 +105,10 @@ class BoardPainter extends CustomPainter {
   }
 
   void _paintGem(Canvas canvas, Rect rect) {
+    final glowPaint = Paint()
+      ..color = GamePalette.gem.withValues(alpha: 0.45)
+      ..maskFilter = MaskFilter.blur(BlurStyle.normal, rect.width * 0.25);
+    canvas.drawCircle(rect.center, rect.width * 0.32, glowPaint);
     final center = rect.center;
     final r = rect.width * 0.32;
     final path = Path()