feat: add red flash on invalid movement (Issue 3)

When user attempts to move out of bounds, show visual feedback:

CSS:
- Added .invalid-move class with red outline and background
- Animation: invalid-flash (500ms ease-out)
- Starts with bright red glow, fades out
- Red outline (3px solid) matches cursor style

Logic:
- flashInvalidMove() function in useCursor
- Triggers when move would go out of bounds (< 0 or > 8)
- Adds invalid-move class to current cell
- Removes class after 500ms
- Passes containerRef to useCursor for DOM access

Cleanup:
- Removed all console.log debugging statements
- Clean code ready for production

UX Flow:
- Press h at left edge → Red flash at current position
- Press j at bottom edge → Red flash at current position
- Press k at top edge → Red flash at current position
- Press l at right edge → Red flash at current position

Issue 3 complete!

src/game.css

@@ -263,3 +263,22 @@
   z-index: 10;
   position: relative;
 }
+
+/* Invalid move flash */
+.sudoku-cell.invalid-move {
+  background-color: rgba(239, 68, 68, 0.4);
+  outline: 3px solid rgb(239, 68, 68);
+  outline-offset: -3px;
+  animation: invalid-flash 0.5s ease-out;
+}
+
+@keyframes invalid-flash {
+  0% {
+    background-color: rgba(239, 68, 68, 0.6);
+    box-shadow: 0 0 20px rgba(239, 68, 68, 0.8);
+  }
+  100% {
+    background-color: rgba(239, 68, 68, 0.4);
+    box-shadow: 0 0 0 rgba(239, 68, 68, 0);
+  }
+}

src/useCursor/useCursor.ts

@@ -2,23 +2,36 @@ import { useState, useEffect } from 'react';
 import type { Coord, CursorManager } from './types';
 import type { GameStatusManager } from '../useGameStatus/types';
 
-export function useCursor(gameStatusManager: GameStatusManager) {
+export function useCursor(gameStatusManager: GameStatusManager, containerRef: React.RefObject<HTMLDivElement | null>) {
   const [position, setPosition] = useState<Coord>({ row: 0, col: 0 });
   const { gameStatus } = gameStatusManager;
 
+  function flashInvalidMove(): void {
+    const container = containerRef.current;
+    if (!container) return;
+
+    const cells = container.querySelectorAll('.sudoku-cell');
+    const index = position.row * 9 + position.col;
+    const targetCell = cells[index];
+    
+    if (targetCell) {
+      targetCell.classList.add('invalid-move');
+      setTimeout(() => {
+        targetCell.classList.remove('invalid-move');
+      }, 500);
+    }
+  }
+
   function move(dRows: number, dCols: number, count: number = 1): void {
     setPosition(currentPos => {
       const newRow = currentPos.row + (dRows * count);
       const newCol = currentPos.col + (dCols * count);
 
-      console.log('🚀 Move attempt:', { currentPos, dRows, dCols, count, newRow, newCol });
-
       if (newRow < 0 || newRow > 8 || newCol < 0 || newCol > 8) {
-        console.log('⛔ Out of bounds, movement blocked');
+        flashInvalidMove();
         return currentPos;
       }
 
-      console.log('✅ Position updated to:', { row: newRow, col: newCol });
       return { row: newRow, col: newCol };
     });
   }
@@ -40,38 +53,24 @@ export function useCursor(gameStatusManager: GameStatusManager) {
   }
 
   useEffect(() => {
-    console.log('🎮 useCursor effect - gameStatus:', gameStatus);
-    
-    if (gameStatus !== 'started') {
-      console.log('⏸️ Not started, skipping key listener');
-      return;
-    }
+    if (gameStatus !== 'started') return;
 
     function handleKeyDown(e: KeyboardEvent) {
       const key = e.key;
-      console.log('⌨️ Key pressed:', key);
 
       if (key === 'h') {
-        console.log('← Move left');
         moveLeft();
       } else if (key === 'j') {
-        console.log('↓ Move down');
         moveDown();
       } else if (key === 'k') {
-        console.log('↑ Move up');
         moveUp();
       } else if (key === 'l') {
-        console.log('→ Move right');
         moveRight();
       }
     }
 
-    console.log('✅ Adding keydown listener');
     window.addEventListener('keydown', handleKeyDown);
-    return () => {
-      console.log('🗑️ Removing keydown listener');
-      window.removeEventListener('keydown', handleKeyDown);
-    };
+    return () => window.removeEventListener('keydown', handleKeyDown);
   }, [gameStatus]);
 
   const cursor: CursorManager = {

src/useCursor/useCursorRender.ts

@@ -12,16 +12,10 @@ export function useCursorRender(
   useEffect(() => {
     if (gameStatus !== 'started') return;
     
-    console.log('🎯 Cursor render triggered - position:', position, 'boardRendered:', boardRendered);
-    
     const container = containerRef.current;
-    if (!container) {
-      console.log('❌ No container ref');
-      return;
-    }
+    if (!container) return;
 
     const cells = container.querySelectorAll('.sudoku-cell');
-    console.log('📊 Found cells:', cells.length);
     
     cells.forEach(cell => {
       cell.classList.remove('cursor');
@@ -29,13 +23,9 @@ export function useCursorRender(
 
     const index = position.row * 9 + position.col;
     const targetCell = cells[index];
-    console.log('🎯 Target cell index:', index, 'cell:', targetCell);
     
     if (targetCell) {
       targetCell.classList.add('cursor');
-      console.log('✅ Added cursor class to cell');
-    } else {
-      console.log('❌ Target cell not found');
     }
   }, [position, containerRef, gameStatus, boardRendered]);
 }

src/useGame.ts

@@ -16,7 +16,7 @@ export function useGame(options: GameOptions = {}, platformHook?: unknown): Game
   const [keyLog, setKeyLog] = useState<{ key: string; timestamp: number }[]>([]);
   const [boardRendered, setBoardRendered] = useState(0);
   
-  const cursor = useCursor(gameStatusManager);
+  const cursor = useCursor(gameStatusManager, containerRef);
   const cursorPosition = cursor.position();
   
   useCursorRender(containerRef, cursorPosition, gameStatus, boardRendered);