feat: implement sentences mode with centered text and slower speed

- Set sentence fall speed to 25 pixels/second
- Replace sentences with longer, realistic ones including commas and punctuation
- Sentences always render centered on screen (80% max width constraint)
- Ship stays centered during sentence mode (no horizontal movement)
- Add punctuation support (. , ! ?) to keyboard input
- Disable auto-tracking for sentence/paragraph modes

example/src/useKeyBindings.ts

@@ -30,9 +30,9 @@ export const useKeyBindings = (gameManager: GameManager) => {
         return;
       }
 
-      // handle letter typing during game
+      // handle letter and punctuation typing during game
       if (gameManager.gameStatus === "started") {
-        if (ev.key.length === 1 && /[a-zA-Z]/.test(ev.key)) {
+        if (ev.key.length === 1 && /[a-zA-Z.,!?]/.test(ev.key)) {
           gameManager.handleTypedLetter(ev.key);
         }
       }