fix(gui-client): show "Loading" in the tray menu while the Client loads (#5553)

Closes #5552

![image](https://github.com/firezone/firezone/assets/13400041/1dd59be6-51e3-48c5-9a12-64dc6ed684b2)

rust/gui-client/src-tauri/src/client/gui.rs

@@ -88,8 +88,10 @@ pub(crate) fn run(
         inject_faults: cli.inject_faults,
     };
 
+    // We can't call `refresh_system_tray_menu` yet because `Controller`
+    // is built inside Tauri's setup
     let tray = SystemTray::new()
-        .with_menu(system_tray_menu::signed_out())
+        .with_menu(system_tray_menu::loading())
         .with_tooltip(TRAY_ICON_TOOLTIP);
 
     tracing::info!("Setting up Tauri app instance...");
@@ -437,7 +439,6 @@ struct Session {
 }
 
 impl Controller {
-    // TODO: Figure out how re-starting sessions automatically will work
     /// Pre-req: the auth module must be signed in
     async fn start_session(&mut self, token: SecretString) -> Result<(), Error> {
         if self.session.is_some() {
@@ -719,6 +720,7 @@ async fn run_controller(
         controller.start_session(token).await?;
     } else {
         tracing::info!("No token / actor_name on disk, starting in signed-out state");
+        controller.refresh_system_tray_menu()?;
     }
 
     if !ran_before::get().await? {

rust/gui-client/src-tauri/src/client/gui/system_tray_menu.rs

@@ -46,6 +46,10 @@ fn get_submenu(res: &ResourceDescription) -> SystemTrayMenu {
     submenu.item(Event::Url(url), address_description)
 }
 
+pub(crate) fn loading() -> SystemTrayMenu {
+    SystemTrayMenu::new().disabled("Loading...")
+}
+
 pub(crate) fn signed_in(user_name: &str, resources: &[ResourceDescription]) -> SystemTrayMenu {
     let mut menu = SystemTrayMenu::new()
         .disabled(format!("Signed in as {user_name}"))