feat: implement digit entry in edit mode (Phase 40.3.3)

- Add setCell method to useBoard to update board state
- Handle digit keys 1-9 in edit mode in useCursor
- Move boardRendered counter to useBoard for proper cursor re-rendering
- Fix golden glow disappearing after digit entry
- Digits display with .user-entered class via renderer

src/useBoard/useBoard.ts

@@ -1,4 +1,4 @@
-import { useRef } from 'react';
+import { useRef, useState } from 'react';
 import { SudokuGenerator } from './SudokuGenerator';
 import { SudokuRenderer } from './SudokuRenderer';
 import type { Difficulty } from './SudokuGenerator';
@@ -10,6 +10,7 @@ export function useBoard() {
   const boardRef = useRef<string>('');
   const solutionRef = useRef<string>('');
   const initialBoardRef = useRef<string>('');
+  const [boardRendered, setBoardRendered] = useState(0);
 
   function generateBoard(difficulty: Difficulty = 'easy') {
     if (!generatorRef.current) {
@@ -41,6 +42,16 @@ export function useBoard() {
     } else {
       rendererRef.current.renderEmpty();
     }
+    
+    setBoardRendered(prev => prev + 1);
+  }
+
+  function setCell(row: number, col: number, value: string): void {
+    const board = boardRef.current.split('');
+    const index = row * 9 + col;
+    board[index] = value;
+    boardRef.current = board.join('');
+    renderBoard();
   }
 
   return {
@@ -50,7 +61,9 @@ export function useBoard() {
     boardRef,
     solutionRef,
     initialBoardRef,
+    boardRendered,
     generateBoard,
     renderBoard,
+    setCell,
   };
 }