Size the rail to the traffic lights instead of guessing

The collapsed rail was a fixed 72px, which left the cluster with more
room on its left than its right. The width now comes from the buttons
themselves - the close button's own inset plus the zoom button's right
edge - so the margin matches on both sides. That inset is macOS's to
choose and has changed between releases, so it is asked for rather than
assumed.

Measured at startup rather than on demand. The answer arrives on the main
thread, and a command waiting for it there deadlocks until the timeout
and silently returns the fallback - which is what the first attempt did.

src-tauri/src/lib.rs

@@ -91,6 +91,9 @@ pub fn run() {
             let chrome = webviews::measure_chrome(&handle);
             *handle.state::<commands::AppState>().chrome.lock().unwrap() = chrome;
 
+            let rail = commands::measure_rail(&handle);
+            *handle.state::<commands::AppState>().rail.lock().unwrap() = rail;
+
             let theme = app.state::<commands::AppState>().cfg().settings.theme;
             commands::apply_window_theme(&app.handle().clone(), &theme);
             Ok(())
@@ -120,6 +123,7 @@ pub fn run() {
             commands::reset_config,
             commands::set_window_chrome,
             commands::chrome_height,
+            commands::rail_width,
             commands::unread_counts,
             commands::focus_window,
             commands::save_password,