feat: implement Phase 40.1 - basic cursor with hjkl movement

Add cursor navigation with hjkl keys (vim-style):

New Module Structure:
- src/useCursor/types.ts: Coord and CursorManager types
- src/useCursor/useCursor.ts: Cursor movement logic
- src/useCursor/useCursorRender.ts: Visual cursor highlight
- src/useCursor/index.ts: Public exports

Cursor Implementation:
- Position tracking with useRef (row/col from 0-8)
- Basic movement: moveLeft, moveRight, moveUp, moveDown
- Bounds checking (prevents moving outside 9x9 grid)
- Keyboard listener for h/j/k/l keys
- Only active when gameStatus === 'started'

Cursor Rendering:
- useCursorRender hook tracks cursor position
- Adds 'cursor' CSS class to current cell
- Removes from previous cell on movement
- Blue highlight with border (rgba(59, 130, 246, 0.3))

Integration:
- src/useGame.ts: Integrate useCursor and useCursorRender
- src/types.ts: Export CursorManager and Coord types
- src/useBoard/types.ts: NEW - BoardManager type
- src/game.css: Add cursor highlight styles

Game Flow:
- Press Space → Game starts → Cursor appears at (0,0)
- Press h/j/k/l → Cursor moves (bounded to grid)
- Visual feedback with blue highlight
- Press Q → Return to waiting state

Phase 40.1 complete. Next: Add number prefixes (5j) and other vim motions.

src/game.css

@@ -254,3 +254,9 @@
       inset 0 0 12px color-mix(in srgb, var(--destructive, #c53b53) 40%, transparent);
   }
 }
+
+/* Cursor highlight */
+.sudoku-cell.cursor {
+  background-color: rgba(59, 130, 246, 0.3);
+  border: 2px solid rgb(59, 130, 246);
+}