…ng (farion1231#4074)

* fix(updater): drive download/install/restart from backend to avoid hang

The 3.16/3.16.1 update flow was frontend-driven: downloadAndInstall()
then relaunchApp(). relaunch() routed through AppHandle::restart(), and
the old WebView had to keep running JS after the .app bundle was already
swapped — an unstable window that could hang the update or leave the old
version running until a manual restart.

Move the whole download -> install -> cleanup -> restart chain into a new
backend command install_update_and_restart, so it no longer depends on the
old WebView running JS after the bundle is swapped.

Platform-aware install ordering (install() behaves differently per OS):
- Windows: install() launches the external installer and exits the process
  internally, so cleanup + single-instance destroy must run before install.
  Surface a recovery hint on failure since the proxy may already be stopped.
- macOS/Linux: install() returns, so install first then cleanup — an install
  failure no longer wrongly stops the proxy / reverts takeover.

Eliminate the restart vs single-instance race: restart_process() destroys
the single-instance lock (remove socket on macOS, ReleaseMutex on Windows)
before tauri::process::restart(), so the freshly spawned process can't
connect to the old listener and exit itself.

Also remove the now-dead frontend update plumbing (relaunchApp,
UpdateHandle, mapUpdateHandle) and surface backend errors in the toast.

Adapted from the original af4271f4 while rebasing onto farion1231#4069: the
ExitRequested handler changes were dropped entirely — the classifier from
farion1231#4069 already routes RESTART_EXIT_CODE to Tauri's default restart flow,
and the original should_restart branch (prevent_exit + async cleanup)
would have reintroduced the window-state deadlock that farion1231#4069 fixed.
install_update_and_restart bypasses ExitRequested entirely, so the two
fixes compose cleanly.

* fix(updater): clear tray icon on the direct-restart update path

restart_process re-execs via tauri::process::restart (spawn + exit(0)),
which skips Tauri's internal cleanup_before_exit and all RunEvent::Exit
plugin hooks. Window state, proxy/live restore and the single-instance
lock were already compensated explicitly; the tray icon was not.

On macOS/Linux the OS drops the status item when the process dies, so
the gap there was cosmetic at most. The real residue risk is the
Windows branch, which never reaches restart_process at all:
update.install() exits the process inside the updater plugin
(std::process::exit(0)), bypassing TrayIcon::drop — no NIM_DELETE is
sent and a stale icon lingers in the shell until hovered, the same
failure remove_tray_icon_before_exit was originally added for on the
quit path.

Call remove_tray_icon_before_exit (set_visible(false), proxied to the
main thread via run_item_main_thread) in restart_process and before
the Windows install. Deliberately not AppHandle::cleanup_before_exit():
it drops tray icons on the calling thread, which is not safe off the
main thread on macOS (NSStatusItem).

…ng (farion1231#4074)

* fix(updater): drive download/install/restart from backend to avoid hang

The 3.16/3.16.1 update flow was frontend-driven: downloadAndInstall()
then relaunchApp(). relaunch() routed through AppHandle::restart(), and
the old WebView had to keep running JS after the .app bundle was already
swapped — an unstable window that could hang the update or leave the old
version running until a manual restart.

Move the whole download -> install -> cleanup -> restart chain into a new
backend command install_update_and_restart, so it no longer depends on the
old WebView running JS after the bundle is swapped.

Platform-aware install ordering (install() behaves differently per OS):
- Windows: install() launches the external installer and exits the process
  internally, so cleanup + single-instance destroy must run before install.
  Surface a recovery hint on failure since the proxy may already be stopped.
- macOS/Linux: install() returns, so install first then cleanup — an install
  failure no longer wrongly stops the proxy / reverts takeover.

Eliminate the restart vs single-instance race: restart_process() destroys
the single-instance lock (remove socket on macOS, ReleaseMutex on Windows)
before tauri::process::restart(), so the freshly spawned process can't
connect to the old listener and exit itself.

Also remove the now-dead frontend update plumbing (relaunchApp,
UpdateHandle, mapUpdateHandle) and surface backend errors in the toast.

Adapted from the original af4271f4 while rebasing onto farion1231#4069: the
ExitRequested handler changes were dropped entirely — the classifier from
farion1231#4069 already routes RESTART_EXIT_CODE to Tauri's default restart flow,
and the original should_restart branch (prevent_exit + async cleanup)
would have reintroduced the window-state deadlock that farion1231#4069 fixed.
install_update_and_restart bypasses ExitRequested entirely, so the two
fixes compose cleanly.

* fix(updater): clear tray icon on the direct-restart update path

restart_process re-execs via tauri::process::restart (spawn + exit(0)),
which skips Tauri's internal cleanup_before_exit and all RunEvent::Exit
plugin hooks. Window state, proxy/live restore and the single-instance
lock were already compensated explicitly; the tray icon was not.

On macOS/Linux the OS drops the status item when the process dies, so
the gap there was cosmetic at most. The real residue risk is the
Windows branch, which never reaches restart_process at all:
update.install() exits the process inside the updater plugin
(std::process::exit(0)), bypassing TrayIcon::drop — no NIM_DELETE is
sent and a stale icon lingers in the shell until hovered, the same
failure remove_tray_icon_before_exit was originally added for on the
quit path.

Call remove_tray_icon_before_exit (set_visible(false), proxied to the
main thread via run_item_main_thread) in restart_process and before
the Windows install. Deliberately not AppHandle::cleanup_before_exit():
it drops tray icons on the calling thread, which is not safe off the
main thread on macOS (NSStatusItem).