Phase 3: session-end UX overhaul + closing-grace cleanup

Promotes the customer-side chat WebSocket to active-session-scoped (driven
by a new `activeSessionProvider`) so home reflects session state in real
time without a per-screen connection. Backend now auto-completes sessions
left in `closing` after a 5-minute grace window so abandoned goodbye flows
don't leave the customer's home permanently locked.

Customer:
- New `activeSessionProvider` (replaces `unread_notifier`) — single source
  of truth for the active session + unread count; polled every 15s.
- Chat WS lifecycle moved to `main.dart` listener on activeSessionProvider.
  Chat screen joins via `connectIfNotConnected`; the new
  `refreshSessionStatus` reconciles flags from the server when re-entering
  an already-connected session (covers missed `sessionClosing`/`sessionExpired`
  WS events).
- Home filters `closing` from the "Sesi Aktif" CTA so a session pending
  goodbye doesn't block "Mulai Curhat".
- Timer-expired UX is a non-dismissible modal (Tutup / Perpanjang) instead
  of an inline bar.
- Early-end goodbye composer gets an amber "Sesi telah ditutup oleh Bestie"
  banner. Goodbye TextEditingController lifted to state so focus changes
  no longer wipe the message.
- Closure provider reset on chat_screen mount to avoid stale
  `ClosureCompleteData` from a previous session leaking into a new view.
- Chat history now lists `closing` sessions with a "Belum ditutup" badge
  that routes to the live chat (goodbye composer) instead of the transcript.

Mitra:
- Same goodbye-controller fix as customer.
- Same chat-history badge + routing for `closing` items.

Backend:
- New `EndedBy.SYSTEM_AUTO_CLOSE` constant.
- `startClosureGraceTimer` extracted in `session-timer.service.js`; wired
  in from `closure.initiateEarlyEnd`, `extension.rejectExtension`, and
  `extension.handleExtensionTimeout`. Cancelled when customer submits
  goodbye.
- Restart recovery (`restoreActiveTimers`) re-arms grace timers and stamps
  any orphaned `closing` rows with `system_auto_close`.
- `getCustomerHistory` / `getMitraHistory` include `closing` alongside
  `completed`; ordering uses `COALESCE(ended_at, created_at)`.

Removed: dead `session_active_screen.dart` (no router entry).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/services/extension.service.js

@@ -1,7 +1,7 @@
 import { getDb } from '../db/client.js'
 import { publish } from '../plugins/valkey.js'
 import { sendToSessionParticipant } from '../plugins/websocket.js'
-import { extendSessionTimer, clearClosureGraceTimer } from './session-timer.service.js'
+import { extendSessionTimer, clearClosureGraceTimer, startClosureGraceTimer } from './session-timer.service.js'
 import { UserType, SessionStatus, ExtensionStatus, TransactionType, WsMessage } from '../constants.js'
 
 const sql = getDb()
@@ -142,6 +142,7 @@ export const respondToExtension = async (extensionId, sessionId, mitraId, accept
       type: WsMessage.SESSION_CLOSING,
       session_id: sessionId,
     })
+    startClosureGraceTimer(sessionId)
   }
 
   return extension
@@ -174,4 +175,5 @@ const timeoutExtension = async (extensionId, sessionId) => {
     type: WsMessage.SESSION_CLOSING,
     session_id: sessionId,
   })
+  startClosureGraceTimer(sessionId)
 }