feat!: remove signal crate for macOS and windows (#7827)

* feat!: remove signal crate

* fix: clippy

* feat(signal): limit the shutdown signal crate to Linux

Upstream now covers termination and session-end events on Windows and
macOS, but Linux has no equivalent, so pre-shutdown cleanup there still
depends on listening for SIGTERM/SIGINT/SIGHUP (#7827). Restore
clash-verge-signal behind a target_os = "linux" gate instead of dropping
it: the dependency lives in a linux-only target section, the crate root
carries an English Linux-only rationale and compiles to nothing
elsewhere, and the Windows backend is gone.

register() now takes the quit flow's unit future: the latch serializes
in-flight cleanups and releases after each attempt, leaving the
exit-or-cancel decision to the quit flow itself.

Verified with cargo check and clippy for aarch64-unknown-linux-gnu, and
cargo check on the host where the crate compiles empty.

src-tauri/src/feat/window.rs

@@ -130,7 +130,7 @@ pub async fn open_or_close_dashboard() {
     logging!(info, Type::Window, "Window toggle result: {result:?}");
 }
 
-pub async fn quit() -> clash_verge_signal::ShutdownOutcome {
+pub async fn quit() {
     logging!(debug, Type::System, "启动退出流程");
     // 设置退出标志
     handle::Handle::global().set_is_exiting();
@@ -165,13 +165,11 @@ pub async fn quit() -> clash_verge_signal::ShutdownOutcome {
             NoticeStatus::AppQuitCoreStopFailed,
             cleanup_result.stop_error.unwrap_or_default(),
         );
-        return clash_verge_signal::ShutdownOutcome::Canceled;
     }
 
     utils::server::shutdown_embedded_server();
     let app_handle = handle::Handle::app_handle();
     app_handle.exit(if cleanup_result.all_success { 0 } else { 1 });
-    clash_verge_signal::ShutdownOutcome::Committed
 }
 
 pub async fn clean_async() -> CleanupResult {