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/l10n/app_en.arb

@@ -3,5 +3,13 @@
   "appTitle": "Block Seasons",
   "play": "Play",
   "settings": "Settings",
-  "comingSoon": "Coming soon"
+  "comingSoon": "Coming soon",
+  "stageClear": "Stage Clear!",
+  "stageFailed": "Stage Failed",
+  "outOfMoves": "Out of moves",
+  "boardFull": "No space left",
+  "watchAdContinue": "Continue (ad)",
+  "plusFiveMoves": "+5 moves (ad)",
+  "giveUp": "Give up",
+  "playAgain": "Play again"
 }