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/ui/screens/game_screen.dart

@@ -5,6 +5,7 @@ import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../../game/engine/game_engine.dart';
+import '../../game/models/grid.dart';
 import '../../l10n/gen/app_localizations.dart';
 import '../../services/audio_service.dart';
 import '../../state/game_session_notifier.dart';
@@ -18,6 +19,7 @@ import '../widgets/hud_widget.dart';
 import '../widgets/piece_painter.dart';
 import '../widgets/season_background.dart';
 import '../widgets/tray_widget.dart';
+import '../widgets/tutorial_overlay.dart';
 
 /// Renders whatever session [gameSessionProvider] holds; callers start the
 /// stage (via SeasonFlowNotifier) before navigating here.
@@ -38,6 +40,8 @@ class _GameScreenState extends ConsumerState<GameScreen>
     duration: const Duration(milliseconds: 350),
   );
 
+  bool _tutorialStartChecked = false;
+
   int? _dragIndex;
   Offset? _dragGlobal;
 
@@ -126,6 +130,10 @@ class _GameScreenState extends ConsumerState<GameScreen>
         audio.play(Sfx.place);
         HapticFeedback.lightImpact();
       }
+      ref.read(tutorialProvider.notifier).onPlaced();
+      if (placement.linesCleared > 0) {
+        ref.read(tutorialProvider.notifier).onLineCleared();
+      }
       final boardBox = _boardBox;
       final stackBox =
           _stackKey.currentContext?.findRenderObject() as RenderBox?;
@@ -175,6 +183,18 @@ class _GameScreenState extends ConsumerState<GameScreen>
       return const Scaffold(body: Center(child: CircularProgressIndicator()));
     }
 
+    final tutorialStep = ref.watch(tutorialProvider);
+    if (!_tutorialStartChecked) {
+      _tutorialStartChecked = true;
+      final flow = ref.read(seasonFlowProvider);
+      if (flow != null &&
+          flow.index == 0 &&
+          !ref.read(saveRepositoryProvider).tutorialDone) {
+        WidgetsBinding.instance.addPostFrameCallback(
+            (_) => ref.read(tutorialProvider.notifier).start());
+      }
+    }
+
     final ghost = _ghost(view);
     final draggedTopLeft = _draggedPieceTopLeft(view);
     final boardBox = _boardBox;
@@ -235,6 +255,17 @@ class _GameScreenState extends ConsumerState<GameScreen>
                   _draggedPieceOverlay(view, draggedTopLeft, boardBox),
                 Positioned.fill(child: EffectsOverlay(key: _effectsKey)),
                 if (view.phase != GamePhase.playing) _resultOverlay(view),
+                if (tutorialStep != null)
+                  Positioned.fill(
+                    child: TutorialOverlay(
+                      step: tutorialStep,
+                      handFrom: _tutorialHandFrom(),
+                      handTo: _tutorialHandTo(view),
+                      onSkip: () => ref.read(tutorialProvider.notifier).skip(),
+                      onDismissHud: () =>
+                          ref.read(tutorialProvider.notifier).dismissHud(),
+                    ),
+                  ),
                 if (Navigator.of(context).canPop())
                   Positioned(
                     top: 4,
@@ -252,6 +283,36 @@ class _GameScreenState extends ConsumerState<GameScreen>
     );
   }
 
+  Offset _tutorialHandFrom() {
+    final stackBox =
+        _stackKey.currentContext?.findRenderObject() as RenderBox?;
+    if (stackBox == null) return Offset.zero;
+    // Tray sits at the bottom; aim at the left slot.
+    final size = stackBox.size;
+    return Offset(size.width * 0.18, size.height - 80);
+  }
+
+  Offset _tutorialHandTo(GameViewState view) {
+    final boardBox = _boardBox;
+    final stackBox =
+        _stackKey.currentContext?.findRenderObject() as RenderBox?;
+    if (boardBox == null || stackBox == null || view.tray.isEmpty) {
+      return Offset.zero;
+    }
+    final geo = BoardGeometry(boardSize: boardBox.size.width);
+    final notifier = ref.read(gameSessionProvider.notifier);
+    for (var y = 0; y < GridState.size; y++) {
+      for (var x = 0; x < GridState.size; x++) {
+        if (notifier.canPlaceAt(0, x, y)) {
+          final local =
+              Offset((x + 0.5) * geo.cellSize, (y + 0.5) * geo.cellSize);
+          return stackBox.globalToLocal(boardBox.localToGlobal(local));
+        }
+      }
+    }
+    return Offset.zero;
+  }
+
   Widget _draggedPieceOverlay(
       GameViewState view, Offset topLeftGlobal, RenderBox boardBox) {
     final stackBox =