fix(session): take down the countdown notification when the tunnel stops

Disconnecting the VPN from Android's own settings left "Revolt VPN
00:28:13" in the shade, frozen, for as long as the phone stayed up - a
notification claiming a protected session over a tunnel that was gone.

updateTimer() re-posts notification id 1 as ongoing and non-dismissible
from the UI process, every tick. The service cancels its own foreground
copy when it stops, but a tick landing just after that re-creates it as
an ordinary notification nobody owns, and nothing ever cancelled it:
NotificationService.reset() only clears the dedupe cache. AGENTS.md
rule 31 describes the cancel this restores.

_doDisconnect now cancels it, after vpnConnection.disconnect() so the
service is already down - cancelling while it is still in the foreground
would be swallowed by the platform.

Verified on an emulator, revoking via Settings -> VPN -> Disconnect:

  before  countdown stranded at 00:28:13 indefinitely
  after   no live notifications

User disconnect still posts no alert, and expiry still posts "Revolt VPN
disconnected" with the countdown gone.

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

android/app/src/main/kotlin/com/paladinvpn/app/MainActivity.kt

@@ -75,6 +75,18 @@ class MainActivity : FlutterActivity() {
                         RevoltAlerts.postDisconnected(this)
                         result.success(null)
                     }
+                    // The countdown re-post is ongoing and non-dismissible, so
+                    // whoever posted it last has to take it down. A tick that
+                    // lands after the service cancelled its own foreground
+                    // notification otherwise strands a frozen countdown that
+                    // claims a protected session forever. Harmless while the
+                    // service is still in the foreground - the platform keeps
+                    // its notification regardless.
+                    "cancelNotification" -> {
+                        (getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager)
+                            .cancel(NOTIFICATION_ID)
+                        result.success(null)
+                    }
                     else -> result.notImplemented()
                 }
             }