chore: Update Gemini model to gemini-2.5-flash-lite and guard fallback; update installation docs

public/INSTALLATION.md

@@ -321,7 +321,7 @@ Error: API key not valid
 Solution:
 1. Verify key in `.env` file
 2. Check key hasn't expired
-3. Test key directly: `curl https://generativelanguage.googleapis.com/v1/models/gemini-pro:generateContent?key=YOUR_KEY`
+3. Test key directly: `curl https://generativelanguage.googleapis.com/v1/models/gemini-2.5-flash-lite:generateContent?key=YOUR_KEY`
 
 ### GitHub Connection Failed
 ```

public/src/services/gemini.js

@@ -13,7 +13,7 @@ class GeminiService {
     if (this.enabled) {
       try {
         this.client = new GoogleGenerativeAI(config.GEMINI_API_KEY);
-        this.model = this.client.getGenerativeModel({ model: 'gemini-pro' });
+        this.model = this.client.getGenerativeModel({ model: 'gemini-2.5-flash-lite' });
         logger.info('Gemini client initialized');
       } catch (error) {
         logger.error('Failed to initialize Gemini client', { error: error.message });