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/api/aiSessions.ts

@@ -15,6 +15,9 @@ import type {
   PickupSessionRequest,
   StatusUpdateRequest,
   StatusUpdateResponse,
+  ChatSessionCreateResponse,
+  ChatMessageRequest,
+  ChatMessageResponse,
 } from '@/types/ai-session'
 
 export const aiSessionsApi = {
@@ -23,6 +26,22 @@ export const aiSessionsApi = {
     return response.data
   },
 
+  async createChatSession(data: AISessionCreateRequest): Promise<ChatSessionCreateResponse> {
+    const response = await apiClient.post<ChatSessionCreateResponse>('/ai-sessions', {
+      ...data,
+      session_type: 'chat',
+    })
+    return response.data
+  },
+
+  async sendChatMessage(sessionId: string, data: ChatMessageRequest): Promise<ChatMessageResponse> {
+    const response = await apiClient.post<ChatMessageResponse>(
+      `/ai-sessions/${sessionId}/chat`,
+      data
+    )
+    return response.data
+  },
+
   async respondToStep(sessionId: string, data: StepResponseRequest): Promise<StepResponseResponse> {
     const response = await apiClient.post<StepResponseResponse>(
       `/ai-sessions/${sessionId}/respond`,
@@ -49,6 +68,7 @@ export const aiSessionsApi = {
 
   async listSessions(params?: {
     status?: string
+    session_type?: string
     skip?: number
     limit?: number
     problem_domain?: string
@@ -104,6 +124,10 @@ export const aiSessionsApi = {
     })
   },
 
+  async deleteSession(sessionId: string): Promise<void> {
+    await apiClient.delete(`/ai-sessions/${sessionId}`)
+  },
+
   async pickupSession(sessionId: string, data: PickupSessionRequest): Promise<StepResponseResponse> {
     const response = await apiClient.post<StepResponseResponse>(
       `/ai-sessions/${sessionId}/pickup`,