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;
       }

src/useGameStatus/useGameStatus.ts

@@ -3,6 +3,7 @@ import type { GameStatus, FallingLetter } from "../types";
 
 const LETTER_SPEED = 100; // pixels per second
 const SPAWN_INTERVAL = 1000; // ms between letter spawns
+const MAX_ACTIVE_LETTERS = 1; // Only one letter on screen at a time
 
 export function useGameStatus(onGameLoopTick?: (deltaTime: number, letters: FallingLetter[]) => void) {
   const [gameStatus, setGameStatus] = useState<GameStatus>("waiting");
@@ -16,8 +17,8 @@ export function useGameStatus(onGameLoopTick?: (deltaTime: number, letters: Fall
     const deltaTime = currentTime - lastFrameTimeRef.current;
     lastFrameTimeRef.current = currentTime;
 
-    // Spawn new letter if enough time has passed
-    if (currentTime - lastSpawnRef.current > SPAWN_INTERVAL) {
+    // Spawn new letter if enough time has passed and no letters active
+    if (currentTime - lastSpawnRef.current > SPAWN_INTERVAL && lettersRef.current.length < MAX_ACTIVE_LETTERS) {
       const randomLetter = String.fromCharCode(65 + Math.floor(Math.random() * 26)); // A-Z
       const randomX = Math.random() * 800;
       const newLetter: FallingLetter = {