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/lib.rs

@@ -230,6 +230,7 @@ pub fn run() -> std::process::ExitCode {
             if let Err(panic) = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
                 resolve::resolve_setup_async();
                 resolve::resolve_setup_sync();
+                #[cfg(target_os = "linux")]
                 resolve::init_signal();
             })) {
                 log_setup_panic("window-core", panic);