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/useCursor/useCursor.ts

@@ -0,0 +1,67 @@
+import { useRef, useEffect } from 'react';
+import type { Coord, CursorManager } from './types';
+import type { GameStatusManager } from '../useGameStatus/types';
+
+export function useCursor(gameStatusManager: GameStatusManager) {
+  const position = useRef<Coord>({ row: 0, col: 0 });
+  const { gameStatus } = gameStatusManager;
+
+  function move(dRows: number, dCols: number, count: number = 1): void {
+    const currentPos = position.current;
+    const newRow = currentPos.row + (dRows * count);
+    const newCol = currentPos.col + (dCols * count);
+
+    if (newRow < 0 || newRow > 8 || newCol < 0 || newCol > 8) {
+      return;
+    }
+
+    position.current = { row: newRow, col: newCol };
+  }
+
+  function moveLeft(count = 1): void {
+    move(0, -1, count);
+  }
+
+  function moveRight(count = 1): void {
+    move(0, 1, count);
+  }
+
+  function moveUp(count = 1): void {
+    move(-1, 0, count);
+  }
+
+  function moveDown(count = 1): void {
+    move(1, 0, count);
+  }
+
+  useEffect(() => {
+    if (gameStatus !== 'started') return;
+
+    function handleKeyDown(e: KeyboardEvent) {
+      const key = e.key;
+
+      if (key === 'h') {
+        moveLeft();
+      } else if (key === 'j') {
+        moveDown();
+      } else if (key === 'k') {
+        moveUp();
+      } else if (key === 'l') {
+        moveRight();
+      }
+    }
+
+    window.addEventListener('keydown', handleKeyDown);
+    return () => window.removeEventListener('keydown', handleKeyDown);
+  }, [gameStatus]);
+
+  const cursor: CursorManager = {
+    position: () => position.current,
+    moveLeft,
+    moveRight,
+    moveUp,
+    moveDown,
+  };
+
+  return cursor;
+}