feat: unified sessions — merge assistant chat into ai_sessions table

Add session_type ('guided'|'chat') and title columns to ai_sessions,
enabling both FlowPilot guided sessions and assistant chat sessions to
live in a single table. This is the foundation for a unified session
history and consistent UX across both interaction modes.

Backend:
- Migration 066: session_type + title columns
- unified_chat_service: chat sessions on ai_sessions with same AI/RAG
- POST /ai-sessions supports session_type='chat' creation
- POST /ai-sessions/{id}/chat for chat messages
- DELETE /ai-sessions/{id} for session deletion
- session_type filter on GET /ai-sessions

Frontend:
- AssistantChatPage rewired to aiSessionsApi (no more assistantChatApi)
- /assistant/:sessionId route for deep-linking
- Session history: type filter pills (All/Guided/Chat), type icons
- Dashboard: both types shown with correct routing and icons
- Fixed glass-border → border-default in dashboard components

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

frontend/src/router.tsx

@@ -194,6 +194,7 @@ export const router = sentryCreateBrowserRouter([
       { path: 'script-builder', element: page(ScriptBuilderPage) },
       { path: 'kb-accelerator', element: page(KBAcceleratorPage) },
       { path: 'assistant', element: page(AssistantChatPage) },
+      { path: 'assistant/:sessionId', element: page(AssistantChatPage) },
       { path: 'flow-assist', element: page(FlowAssistPage) },
       { path: 'pilot', element: page(FlowPilotSessionPage) },
       { path: 'pilot/:sessionId', element: page(FlowPilotSessionPage) },