fix(service): surface a service core that stopped for good

When the service watchdog gave up or the core vanished, the owner
monitor marked the core stopped without telling the user, and on macOS
left the system proxy on the dead mixed port (#8029).

- On SameOwnerFailure, clear the proxy through the helper while the
  session is still ours; displaced or unreachable owners still leave the
  machine-wide setting alone.
- Retain the stop as a core failure with its own message, recorded
  under the lifecycle lock and shown now or on next focus; a running
  core resets the notice dedup.
- Log each watchdog restart the service reports, since launchd discards
  the service's own log.

src-tauri/src/cmd/service.rs

@@ -115,7 +115,7 @@ pub fn take_service_fallback_notice() -> bool {
 }
 
 #[tauri::command]
-pub fn get_core_startup_error() -> Option<String> {
+pub fn get_core_startup_error() -> Option<crate::core::manager::CoreFailure> {
     crate::core::CoreManager::global().get_startup_error()
 }