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>

test/game/engine/game_engine_test.dart

@@ -111,6 +111,8 @@ void main() {
       final monoIndex = engine.tray.indexWhere((p) => p.id == 'mono');
       final result = engine.tryPlace(monoIndex, 0, 3)!;
       expect(result.linesCleared, 1);
+      expect(result.clearedRows, [3]);
+      expect(result.clearedCols, isEmpty);
       // 1 cell + round(100 * 1.5) = 151
       expect(engine.score, 151);
       expect(engine.grid.occupiedCount, 0);