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/utils/resolve/mod.rs

@@ -20,7 +20,6 @@ use crate::{
     utils::{init, server, window_manager::WindowManager},
 };
 use clash_verge_logging::{Type, logging, logging_error};
-use clash_verge_signal;
 
 #[cfg(target_os = "macos")]
 pub mod dns;
@@ -189,6 +188,8 @@ async fn init_silent_updater() {
     logging!(info, Type::Setup, "Silent updater initialized");
 }
 
+// Upstream has no session-end signal support on Linux; see crates/clash-verge-signal.
+#[cfg(target_os = "linux")]
 pub(crate) fn init_signal() {
     logging!(debug, Type::Setup, "Initializing signal handlers...");
     clash_verge_signal::register(feat::quit);