feat: first-play interactive tutorial overlay

Add TutorialOverlay widget (dim veil, message bubble, animated hand on
dragPiece step, skip button) and wire it into game_screen: start on
flow.index==0 when tutorialDone is false, forward onPlaced/onLineCleared
events unconditionally from fxTick handler, and compute hand-path
coordinates from board/tray RenderBox geometry.

lib/l10n/app_en.arb

@@ -37,5 +37,10 @@
         "type": "int"
       }
     }
-  }
+  },
+  "skip": "Skip",
+  "gotIt": "Got it!",
+  "tutorialDrag": "Drag a block onto the board!",
+  "tutorialClear": "Fill a row or column to clear it!",
+  "tutorialHud": "Hit the goal before you run out of moves. Your turn!"
 }

lib/l10n/app_ko.arb

@@ -16,5 +16,10 @@
   "streakMilestone": "{days}일 연속 플레이! 대단해요!",
   "almostThere": "{percent}% 달성!",
   "seasonLabel": "SEASON",
-  "seasonStages": "{count}개 스테이지"
+  "seasonStages": "{count}개 스테이지",
+  "skip": "건너뛰기",
+  "gotIt": "알겠어요!",
+  "tutorialDrag": "블록을 보드로 끌어다 놓아보세요!",
+  "tutorialClear": "가로나 세로 한 줄을 채우면 사라져요!",
+  "tutorialHud": "이동 횟수가 끝나기 전에 목표를 달성하세요. 이제 직접!"
 }