feat: add model selector, skills/memory pages, and config management

- Add model selector in sidebar that discovers models from auth.json credential pool
- Add per-session model display (badge in chat header and session list)
- Add skills browser page and memory editor page
- Add BFF routes for skills, memory, and config model management
- Model switching updates config.yaml provider field to bypass env auto-detection
- Refactor Settings page, simplify ChatInput with file upload
- Add attachment upload support via BFF /upload endpoint

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/api/chat.ts

@@ -10,6 +10,7 @@ export interface StartRunRequest {
   instructions?: string
   conversation_history?: ChatMessage[]
   session_id?: string
+  model?: string
 }
 
 export interface StartRunResponse {