saved sessions: replace transient history with persistent DB storage and auto-save

- Add saved_sessions table and CRUD endpoints (save, list, resume, update, delete)
- Save is instant (no LLM summarization), stores exact conversation with tool calls
- Resume loads full message history into chat UI, sends transcript to agent on first message
- Auto-save updates DB after every agent response once a session is saved
- Delete old filesystem-based session/group management (sessions router, useChatSessions, useChatGroups)
- Clean up ChatHeader, SessionList, ChatDetailPanel for saved sessions flow

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

src/apps/officer-web/state/useInitialData.ts

@@ -1,4 +1,4 @@
-import { useChatSessions } from 'state/useChatSessions';
+import { useSavedSessions } from 'state/useSavedSessions';
 import { usePlans } from 'state/usePlans';
 import { useSettings } from 'state/useSettings';
 import { usePiModels } from 'state/useModels';
@@ -6,7 +6,7 @@ import { useAccessPolicy } from 'state/useAccessPolicy';
 import { useColorModeSync } from './useThemeSync';
 
 export const useInitialData = () => {
-  const { sessions } = useChatSessions();
+  const { sessions } = useSavedSessions();
   const { plans } = usePlans();
   const { settings } = useSettings();
   usePiModels();