fix(session): alert on every unsolicited native stop, not just the deadline

Second review pass found that VpnStatus.error is unreachable mid-session:
AppConfigs.V2RAY_STATES has no error state and the plugin's stateName when()
ends in else -> "DISCONNECTED", so Dart's unknown -> error mapping is dead
code on Android. Every native stop therefore arrived as a plain disconnect,
and anything other than the expiry deadline - an Xray or TUN setup failure, a
mid-session fail-closed deadline arm - left the user on their real IP with no
notification and no message. That is the exact hole this feature exists to
close.

stopAll() is the single funnel for every native stop, so the alert moves
there: it fires when the stop interrupted a live tunnel and the user did not
ask for it. isRunning is captured before cleanup() clears it, which keeps a
failed connect silent - it never had a tunnel to interrupt. STOP_SERVICE
(the notification's Disconnect action) and onRevoke pass userRequested: in
the revoke case the user switched VPNs or withdrew consent themselves, and
may well be protected by the app that took over.

Also from the pass:
- re-arming now needs the deadline to move by more than 15 s, so round-trip
  jitter cannot cancel and re-post a warning that is already on screen
- a warning delivered with under a minute left is dropped rather than
  announcing "5 minutes"; the disconnected alert follows anyway
- the _doDisconnect comment no longer claims endedMessage posts the alert
- CHANGELOG: the alert does now cover the tunnel stopping unexpectedly

flutter analyze clean; debug APK builds with the Kotlin recompiled.

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

CHANGELOG.md

@@ -12,8 +12,8 @@ Versions follow [semver](https://semver.org/): MAJOR.MINOR.PATCH
   off the same deadline it already enforces, so the warning still arrives when
   the app is closed, and a support top-up moves it with the deadline.
 - **"Disconnected — not protected" notification** when a session ends without
-  you asking: the 2-hour expiry, the data limit, or the server ending the
-  session.
+  you asking: the 2-hour expiry, the data limit, the server ending the
+  session, or the tunnel stopping unexpectedly.
 - **SOCKS5 mode now asks for the notification permission.** It never did,
   because it never raises the VPN consent dialog, so on Android 13+ these
   alerts would have been dropped before they were shown.