fix(updater): drive download/install/restart from backend to avoid hang (#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 #4069: the
ExitRequested handler changes were dropped entirely — the classifier from
#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 #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).

src-tauri/src/lib.rs

@@ -1165,6 +1165,7 @@ pub fn run() {
             commands::get_log_config,
             commands::set_log_config,
             commands::restart_app,
+            commands::install_update_and_restart,
             commands::check_for_updates,
             commands::is_portable_mode,
             commands::copy_text_to_clipboard,
@@ -1645,7 +1646,7 @@ pub async fn cleanup_before_exit(app_handle: &tauri::AppHandle) {
 /// 触发 tray-icon 内部的 `remove_tray_icon` → `Shell_NotifyIconW(NIM_DELETE)`，
 /// 在进程结束前干净地把图标摘掉。其它平台 `set_visible(false)` 也是
 /// 正常的隐藏/移除语义，作为跨平台兜底也安全。
-fn remove_tray_icon_before_exit(app_handle: &tauri::AppHandle) {
+pub(crate) fn remove_tray_icon_before_exit(app_handle: &tauri::AppHandle) {
     if let Some(tray) = app_handle.tray_by_id(tray::TRAY_ID) {
         if let Err(e) = tray.set_visible(false) {
             log::warn!("退出时移除托盘图标失败: {e}");
@@ -1962,6 +1963,32 @@ pub fn save_window_state_before_exit(app_handle: &tauri::AppHandle) {
     }
 }
 
+/// 主动释放 single-instance 锁。
+///
+/// macOS single-instance 使用 `/tmp/{identifier}.sock`。我们有若干路径会直接
+/// `std::process::exit(0)`，不会触发插件挂在 `RunEvent::Exit` 上的清理钩子。
+/// 重启前主动 destroy 可以避免新进程误连旧 listener 后自行退出。
+pub fn destroy_single_instance_lock(app_handle: &tauri::AppHandle) {
+    #[cfg(any(target_os = "macos", target_os = "windows", target_os = "linux"))]
+    tauri_plugin_single_instance::destroy(app_handle);
+}
+
+/// 清理托盘图标、释放 single-instance 锁后重启当前应用。
+///
+/// 直接走 `tauri::process::restart`（spawn 新进程 + `exit(0)`），不经过事件
+/// 循环退出，因此 Tauri 内部的 `cleanup_before_exit` 和各插件的
+/// `RunEvent::Exit` 钩子都不会执行。需要的清理由调用方与本函数显式补偿：
+/// 窗口状态、代理/Live 恢复（调用方）；托盘图标、single-instance 锁（本函数）。
+///
+/// 有意不调 `AppHandle::cleanup_before_exit()`：它会在调用线程上 Drop 托盘
+/// 图标，而 macOS 的 NSStatusItem 操作要求主线程；`set_visible(false)` 走
+/// `run_item_main_thread` 代理，跨线程安全（见 `remove_tray_icon_before_exit`）。
+pub fn restart_process(app_handle: &tauri::AppHandle) -> ! {
+    remove_tray_icon_before_exit(app_handle);
+    destroy_single_instance_lock(app_handle);
+    tauri::process::restart(&app_handle.env());
+}
+
 #[cfg(test)]
 mod tests {
     use super::{classify_exit_request, ExitRequestAction};