Add playable core UI: board painter, drag-and-drop, HUD, result overlay

CustomPainter board with gems/ghost/clear-flash, finger-lifted drag
with snap preview, combo text effect, HUD chips, phase overlays with
rescue stubs, demo stage. E2E widget test drives a real drag gesture.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/ui/theme/palette.dart

@@ -0,0 +1,26 @@
+import 'package:flutter/material.dart';
+
+/// Season-themeable color set. Season 1 default: vivid candy tones on a
+/// deep navy board.
+class GamePalette {
+  static const boardBackground = Color(0xFF151A2E);
+  static const emptyCell = Color(0xFF222A45);
+  static const boardBorder = Color(0xFF2E3858);
+
+  static const tileColors = <Color>[
+    Color(0xFF5B7FFF), // blue
+    Color(0xFFFF6B6B), // red
+    Color(0xFFFFC94D), // yellow
+    Color(0xFF4DD599), // green
+    Color(0xFFB980FF), // purple
+    Color(0xFFFF8FAB), // pink
+    Color(0xFF4DC9E6), // cyan
+    Color(0xFFFF9A5B), // orange
+  ];
+
+  static Color tile(int colorId) => tileColors[colorId % tileColors.length];
+
+  static const gem = Color(0xFF7CF5FF);
+  static const ghostLegal = Color(0x66FFFFFF);
+  static const ghostIllegal = Color(0x55FF5252);
+}