feat: LAN update check on startup with notify banner (stage 2)

Read manifest.json from the configured update_source and show a dismissable
banner when a newer version is available. Notify-only — no install yet.

- services/update_service.py: version compare + check_for_update (pure, tested)
- config: add update_source key (empty = no check)
- main_window: top banner, background-thread check; open folder via os.startfile
  (QDesktopServices.openUrl mishandles file:// folder URLs — ShellExecute err 2)
- tests: +15 covering version compare and check_for_update branches
- docs 02/05/10 + tasks 17.16

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/services/config_service.py

@@ -11,6 +11,7 @@ DEFAULT_CONFIG = {
     "last_print_dir": "",
     "last_template": "",       # name of the last-selected template
     "last_batch_mode": "full_combo",  # BatchMode value of the last-used mode
+    "update_source": "",       # LAN folder holding manifest.json (empty = no update check)
 }
 
 _CONFIG_FILENAME = "app_config.json"