feat: add spinning favicon during processing and implement rate limiting for Gemini API requests

- Introduced a new hook `useSpinningFavicon` to animate the favicon when processing is ongoing.
- Updated the `Header` component to utilize the new spinning favicon feature.
- Added a rate limit delay of 100ms between requests to the Gemini API in `GeminiAgent`.

src/services/worker/GeminiAgent.ts

@@ -264,6 +264,9 @@ export class GeminiAgent {
 
     const url = `${GEMINI_API_URL}/${model}:generateContent?key=${apiKey}`;
 
+    // Rate limit delay - Gemini API requires spacing between requests
+    await new Promise(resolve => setTimeout(resolve, 100));
+
     const response = await fetch(url, {
       method: 'POST',
       headers: {