feat: multi-turn conversations and Claude fallback for Gemini provider

Major improvements to Gemini provider:

**Shared Conversation History**
- Add ConversationMessage interface for provider-agnostic history
- Both Claude and Gemini agents read/write shared conversationHistory
- Context persists across provider switches via claudeSessionId linkage

**Multi-Turn Gemini API**
- Replace stateless single-query with full conversation context
- queryGeminiMultiTurn() sends entire history for coherent responses
- Maps 'assistant' role to 'model' for Gemini API compatibility

**Automatic Fallback to Claude**
- Detect rate limits (429), server errors (5xx), network failures
- Fall back to Claude SDK when Gemini API fails
- Reset 'processing' messages to 'pending' before fallback

**Mid-Session Provider Switching**
- Track currentProvider on ActiveSession
- Provider changes take effect after current generator finishes
- Avoids race conditions from aborting active generators

Files changed:
- worker-types.ts: Add ConversationMessage, currentProvider tracking
- GeminiAgent.ts: Multi-turn queries, fallback logic
- SDKAgent.ts: Capture messages to shared history
- SessionManager.ts: Initialize new session fields
- SessionRoutes.ts: Provider selection and switching logic
- worker-service.ts: Wire up fallback agent dependency

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/services/worker-service.ts

@@ -85,6 +85,7 @@ export class WorkerService {
     this.sseBroadcaster = new SSEBroadcaster();
     this.sdkAgent = new SDKAgent(this.dbManager, this.sessionManager);
     this.geminiAgent = new GeminiAgent(this.dbManager, this.sessionManager);
+    this.geminiAgent.setFallbackAgent(this.sdkAgent);  // Enable fallback to Claude on Gemini API failure
     this.paginationHelper = new PaginationHelper(this.dbManager);
     this.settingsManager = new SettingsManager(this.dbManager);
     this.sessionEventBroadcaster = new SessionEventBroadcaster(this.sseBroadcaster, this);