Phase 4 §1/§5: notif banner detection on API <33 + chat-delivery WS→FCM lifecycle

§1 notif banner: permission_handler v11 returns granted unconditionally
for Permission.notification on Android <13 because POST_NOTIFICATIONS
didn't exist as a runtime permission. Result: SHome1st amber "notifikasi
off" banner never showed on API 24-32 even when the user toggled
notifications off in Settings → Apps. Add a
NotificationManagerCompat.areNotificationsEnabled() pre-check via
flutter_local_notifications (works from API 19+) so the banner reflects
the real OS state on older Android.

§5 chat delivery: the contract is "WS when foreground, FCM when
background", but the previous build only honoured (1) — Android keeps
the TCP socket alive after the Dart isolate is paused, so backend's
`socket.readyState === 1` check returned true and FCM never fired.
Fix has five parts (all required together):

 1. Customer-side lifecycle observer in client_app/main.dart closes
    chatProvider's WS on paused/detached, reconnects on resumed.
 2. `_appPaused` gate in main.dart suppresses the activeSessionProvider
    listener's auto-reconnect (15s poll in active_session_notifier
    would otherwise re-open the WS the next tick after the observer
    closed it — defeating the fallback).
 3. Mitra-side lifecycle observer in mitra_app/main.dart stashes
    `_pausedChatSessionId`, calls mitraChatProvider.disconnect(), and
    re-issues connect(saved) on resumed.
 4. MitraChat gains a `_connectedSessionId` field + getter so the
    observer in step 3 can read it back across disconnect (disconnect
    clears it; the next connect overwrites it).
 5. SearchingScreen resets pairingProvider when entering with a new
    draft.paymentId — previously it retained PairingActiveData with
    the *old* sessionId after a session ended, and the next pairing
    flow navigated straight to that completed session showing
    "Sesi sudah berakhir".

Backend additions under /internal/_test/* for assertion harness:
inspectSessionWsState + GET /ws-connection-state,
POST /send-chat-message-as-mitra (with delivered_via),
POST /send-chat-message-as-customer (with delivered_via),
POST /send-fcm-chat-message (raw FCM dispatch).

Maestro coverage:
 - ts-customer-05-01: mitra → customer message when customer is
   backgrounded → delivered_via=fcm.
 - ts-customer-05-02: customer → mitra message when mitra is
   backgrounded → delivered_via=fcm.
 - ts-customer-01-01: §1 notif-denied banner on home. Documented
   precondition: mitra must be force-stopped or backgrounded on the
   chat screen before 05-02 runs (Maestro can only drive one --udid
   per run; mitra-side lifecycle observer end-to-end is deferred).

Helper scripts under client_app/.maestro/scripts/:
inspect_ws_state.js, assert_ws_state.js,
send_chat_message_as_mitra.js, assert_delivered_via.js (takes
SENDER=mitra|customer to route to the matching backend endpoint).

README_section_05.md documents the test plan, helper scripts, and the
deferred mitra-side maestro driving. Both apps tested manually on
API 28 AVDs where FCM delivery is sub-second; API 24 has 5-30 min
heartbeats that make it impractical for FCM-related testing.

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

client_app/.maestro/scripts/send_chat_message_as_mitra.js

@@ -0,0 +1,26 @@
+// Send a chat message AS the mitra of the given session via the dev-only
+// /internal/_test/send-chat-message-as-mitra endpoint. Goes through the
+// real chat.service.sendMessage code path. Returns which transport
+// carried the message — `websocket` if the customer's WS was alive at
+// dispatch time, `fcm` otherwise.
+//
+// Env:
+//   SESSION_ID            (required)
+//   CONTENT               (required, message body)
+//   BACKEND_INTERNAL_URL  (optional, default http://localhost:3001)
+const sessionId = SESSION_ID
+const content = CONTENT
+const url = BACKEND_INTERNAL_URL || 'http://localhost:3001'
+if (!sessionId) throw new Error('SESSION_ID env not set')
+if (!content) throw new Error('CONTENT env not set')
+
+const resp = http.post(`${url}/internal/_test/send-chat-message-as-mitra`, {
+  headers: { 'Content-Type': 'application/json' },
+  body: JSON.stringify({ session_id: sessionId, content }),
+})
+if (resp.status !== 200) {
+  throw new Error(`send-chat-message-as-mitra failed (${resp.status}): ${resp.body}`)
+}
+const data = json(resp.body)
+output.MESSAGE_ID = data.message_id
+output.DELIVERED_VIA = data.delivered_via