feat(session): hand the session back when the user disconnects

The client never called /session/stop - the string appears nowhere in
lib/ or local_packages/, on this branch or on main - while the server has
implemented the route all along (hivemind/api.py:237) and AGENTS.md rule
33 documents its ordering. Across a full device test session, two user
disconnects and several expiries, the control plane logged zero calls.

So a disconnect left the VLESS client provisioned in the Reality inbound
and the session dict in memory and sessions.json until its window ran
out. Anyone holding that UUID could keep tunnelling for the remainder.
Users were not locked out of their quota - _handle_main replaces the
session with a fresh full-length one on the next connect - so this is
hygiene rather than a user-facing bug.

Only SessionTimer.disconnect() calls it, which is reached only from the
connect button. Expiry, the hard cap and a server-ended session are
already gone server-side, and the route takes nothing but a device_id as
proof of ownership, so no error path may reach it. Unawaited and capped
at 3 s, so an unreachable API cannot delay the teardown, and every
failure is swallowed: the session expires on its own regardless.

Verified against a mock control plane:

  user disconnect  POST /api/session/stop -> session removed
  expiry           no call; alert still posted

Co-Authored-By: Claude Opus 5 <[email protected]>

lib/logic/session_timer.dart

@@ -241,6 +241,10 @@ class SessionTimer extends ChangeNotifier with WidgetsBindingObserver {
   }
 
   Future<void> disconnect({String reason = 'User requested'}) async {
+    // Only here, not in _doDisconnect: this is the one path the user actually
+    // asked for. Unawaited so a slow or unreachable API never holds up the
+    // teardown.
+    unawaited(HivemindService.releaseSession());
     await _doDisconnect(reason);
   }