feat: add edit mode rules and use existing CSS classes

Rules Implemented:
1. Can only enter edit mode on empty cells or user-entered cells
2. Pressing 'i' on a given cell (from algo) triggers red flash
3. Edit mode blocked on cells with 'given' class

Visual Changes:
- Use existing 'active' class for normal mode cursor (blue glow)
- Use existing 'editing' class for edit mode (yellow/amber glow)
- Removed custom cursor/cursor-edit classes
- Matches original CSS design from src_old

Cell Detection:
- Check if cell has 'given' class before entering edit mode
- Given cells: From puzzle generation (immutable)
- Empty or user-entered cells: Can be edited

Flow:
1. Navigate to given cell → Press 'i' → Red flash (blocked)
2. Navigate to empty cell → Press 'i' → Yellow editing glow
3. Navigate to user-entered cell → Press 'i' → Yellow editing glow
4. Press Escape → Return to blue active glow

Phase 40.3.2 complete.

src/game.css

@@ -255,31 +255,6 @@
   }
 }
 
-/* Cursor highlight - Normal mode */
-.sudoku-cell.cursor {
-  background-color: rgba(59, 130, 246, 0.3);
-  outline: 3px solid rgb(59, 130, 246);
-  outline-offset: -3px;
-  z-index: 10;
-  position: relative;
-}
-
-/* Cursor highlight - Edit mode */
-.sudoku-cell.cursor-edit {
-  background-color: rgba(34, 197, 94, 0.3);
-  outline: 3px solid rgb(34, 197, 94);
-  animation: edit-pulse 1.5s ease-in-out infinite;
-}
-
-@keyframes edit-pulse {
-  0%, 100% {
-    outline-color: rgb(34, 197, 94);
-  }
-  50% {
-    outline-color: rgb(74, 222, 128);
-  }
-}
-
 /* Invalid move flash */
 .sudoku-cell.invalid-move {
   background-color: rgba(239, 68, 68, 0.4);

src/useCursor/useCursor.ts

@@ -102,6 +102,18 @@ export function useCursor(gameStatusManager: GameStatusManager, containerRef: Re
       }
 
       if (key === 'i') {
+        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.contains('given')) {
+          flashInvalidMove();
+          return;
+        }
+
         setMode('edit');
         return;
       }

src/useCursor/useCursorRender.ts

@@ -19,7 +19,7 @@ export function useCursorRender(
     const cells = container.querySelectorAll('.sudoku-cell');
     
     cells.forEach(cell => {
-      cell.classList.remove('cursor', 'cursor-edit');
+      cell.classList.remove('active', 'editing');
     });
 
     const index = position.row * 9 + position.col;
@@ -27,9 +27,9 @@ export function useCursorRender(
     
     if (targetCell) {
       if (mode === 'edit') {
-        targetCell.classList.add('cursor', 'cursor-edit');
+        targetCell.classList.add('editing');
       } else {
-        targetCell.classList.add('cursor');
+        targetCell.classList.add('active');
       }
     }
   }, [position, mode, containerRef, gameStatus, boardRendered]);