feat(02-01): implement config loader and Gemini API client

- Config loader: fetches/caches public/config.json with GeminiConfig interface
- generateText: Gemini REST with 3x exponential backoff retry, returns string|null
- generateImage: reference images as base64 inlineData, responseModalities: ["IMAGE"]
- Rate limiting: checkRateLimit/incrementApiCall with daily reset in Settings store
- Extended Settings type with apiCallsToday and lastApiCallDate fields
- 18 tests across config.test.ts and gemini.test.ts, all passing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/types.ts

@@ -48,6 +48,8 @@ export interface Settings {
 	id: 1; // singleton
 	audioEnabled: boolean;
 	apiKey: string;
+	apiCallsToday: { text: number; image: number };
+	lastApiCallDate: string; // ISO date "YYYY-MM-DD"
 }
 
 // Level definition (used in game/levels.ts)