feat: implement startingLevel option and remove INITIAL_FPS constant

- Remove INITIAL_FPS constant entirely - no longer exists
- Add startingLevel to GameOptions (defaults to 1)
- FPS now equals currentLevel (1:1 ratio)
- Points per food always equals currentLevel
- Game can start at any level with appropriate speed/scoring

src/types.ts

@@ -32,6 +32,7 @@ export type GameOptions = {
   cols: number;
   rows: number;
   timeLimit?: number;
+  startingLevel?: number;
 };
 
 // ============================================================================