fix(updater): keep auto-updater alive across watcher restarts (v0.4.1)

Saving the Settings dialog calls _restart_watcher, which set+replaced the
shared stop_event the updater loop also keyed off — so a settings save
silently killed the auto-updater thread (race: survived one save, died on
the next). Heartbeats kept flowing but the agent stopped checking for /
applying updates until relaunch.

Give the updater its own app-lifetime event (_app_stop), set only on
Exit / when applying an update and untouched by watcher restarts. Adds
test_thor_tray.py (4 lifecycle tests). Bump to v0.4.1.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

installer.iss

@@ -3,7 +3,7 @@
 
 [Setup]
 AppName=Thor Watcher
-AppVersion=0.4.0
+AppVersion=0.4.1
 AppPublisher=Terra-Mechanics Inc.
 DefaultDirName={pf}\ThorWatcher
 DefaultGroupName=Thor Watcher