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/theme/palette.dart

@@ -1,5 +1,7 @@
 import 'package:flutter/material.dart';
 
+import '../../game/models/season.dart';
+
 /// Season-themeable color set. Season 1 default: vivid candy tones on a
 /// deep navy board.
 class GamePalette {
@@ -24,3 +26,26 @@ class GamePalette {
   static const ghostLegal = Color(0x66FFFFFF);
   static const ghostIllegal = Color(0x55FF5252);
 }
+
+/// Resolved per-season colors for the UI layer. Built from a SeasonTheme;
+/// falls back to the GamePalette constants.
+class ThemeColors {
+  ThemeColors(SeasonTheme theme)
+      : gradient = [for (final c in theme.backgroundGradient) Color(c)],
+        accent = Color(theme.accentColor),
+        particleType = theme.particleType,
+        board = theme.boardTint != null
+            ? Color(theme.boardTint!)
+            : GamePalette.boardBackground,
+        tiles = theme.tilePalette != null
+            ? [for (final c in theme.tilePalette!) Color(c)]
+            : GamePalette.tileColors;
+
+  final List<Color> gradient;
+  final Color accent;
+  final String particleType;
+  final Color board;
+  final List<Color> tiles;
+
+  Color tile(int colorId) => tiles[colorId % tiles.length];
+}