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!"
 }