feat: implement falling letters spawning from top

- Letters spawn at random horizontal positions every 1 second
- Letters fall at 100px/second from top to bottom
- Random A-Z letters rendered in white monospace font
- Fix game loop to start properly when startGame called
- Remove letters that exit bottom of screen

src/useGameStatus/useGameStatus.ts

@@ -1,16 +1,67 @@
-import { useState } from "react";
-import type { GameStatus } from "../types";
+import { useState, useRef, useCallback } from "react";
+import type { GameStatus, FallingLetter } from "../types";
 
-export function useGameStatus() {
+const LETTER_SPEED = 100; // pixels per second
+const SPAWN_INTERVAL = 1000; // ms between letter spawns
+
+export function useGameStatus(onGameLoopTick?: (deltaTime: number, letters: FallingLetter[]) => void) {
   const [gameStatus, setGameStatus] = useState<GameStatus>("waiting");
+  const gameLoopRef = useRef<number | null>(null);
+  const lastFrameTimeRef = useRef<number>(0);
+  const lettersRef = useRef<FallingLetter[]>([]);
+  const lastSpawnRef = useRef<number>(0);
 
-  const startGame = () => {
+  // Game loop
+  const gameLoop = useCallback((currentTime: number) => {
+    const deltaTime = currentTime - lastFrameTimeRef.current;
+    lastFrameTimeRef.current = currentTime;
+
+    // Spawn new letter if enough time has passed
+    if (currentTime - lastSpawnRef.current > SPAWN_INTERVAL) {
+      const randomLetter = String.fromCharCode(65 + Math.floor(Math.random() * 26)); // A-Z
+      const randomX = Math.random() * 800;
+      const newLetter: FallingLetter = {
+        id: `${currentTime}-${randomX}`,
+        letter: randomLetter,
+        x: randomX,
+        y: 0,
+      };
+      lettersRef.current.push(newLetter);
+      lastSpawnRef.current = currentTime;
+    }
+
+    // Update letter positions
+    lettersRef.current.forEach(letter => {
+      letter.y += (LETTER_SPEED * deltaTime) / 1000;
+    });
+
+    // Remove letters that went off screen
+    lettersRef.current = lettersRef.current.filter(letter => letter.y < 600);
+
+    // Call tick callback if provided
+    if (onGameLoopTick) {
+      onGameLoopTick(deltaTime, lettersRef.current);
+    }
+
+    // Continue loop
+    gameLoopRef.current = requestAnimationFrame(gameLoop);
+  }, [onGameLoopTick]);
+
+  const startGame = useCallback(() => {
+    lastFrameTimeRef.current = performance.now();
+    lastSpawnRef.current = performance.now();
+    lettersRef.current = [];
     setGameStatus("started");
-  };
+    gameLoopRef.current = requestAnimationFrame(gameLoop);
+  }, [gameLoop]);
 
-  const quitGame = () => {
+  const quitGame = useCallback(() => {
     setGameStatus("waiting");
-  };
+    if (gameLoopRef.current !== null) {
+      cancelAnimationFrame(gameLoopRef.current);
+      gameLoopRef.current = null;
+    }
+  }, []);
 
   return {
     gameStatus,