feat: enforce one active letter at a time (character mode)

- Add MAX_ACTIVE_LETTERS constant set to 1
- Only spawn new letter if no letters are currently active
- Prepare for future game modes (word, sentence, paragraph)
- Change keybindings to use Escape instead of Q

example/src/App.tsx

@@ -30,7 +30,7 @@ function App() {
 
       <div className="text-center text-sm text-muted-foreground">
         {gameStatus === "waiting" && <p>Press <kbd className="px-2 py-1 bg-muted rounded">space</kbd> to start</p>}
-        {gameStatus === "started" && <p>Game running...</p>}
+        {gameStatus === "started" && <p>Type letters • Press <kbd className="px-2 py-1 bg-muted rounded">esc</kbd> to quit</p>}
         {gameStatus === "game-over" && <p>Game Over! Press <kbd className="px-2 py-1 bg-muted rounded">space</kbd> to restart</p>}
         {gameStatus === "game-won" && <p>You Won! Press <kbd className="px-2 py-1 bg-muted rounded">space</kbd> to restart</p>}
       </div>

example/src/useKeyBindings.ts

@@ -25,7 +25,7 @@ export const useKeyBindings = (gameManager: GameManager) => {
       if (gameStatus === "waiting") return;
 
       // quit
-      if (ev.key === "q" || ev.key === "Q") {
+      if (ev.key === "Escape") {
         quitGame();
         return;
       }