feat(gui): --first-run flag + firstrun::run stub for Phase 10

argv is read manually before adw::Application sees it (GTK's option parser
rejects unknown long flags), then the filtered list is passed to
app.run_with_args. Stub exits cleanly with a stderr breadcrumb so a
developer who passes --first-run before Phase 10 lands doesn't get a
stuck process.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

gui/src/main.rs

@@ -5,6 +5,7 @@ mod app_context;
 mod bus;
 mod enroll_dialog;
 mod error;
+mod firstrun;
 mod keys_page;
 
 const APP_ID: &str = "io.dangerousthings.AuthForge";
@@ -20,14 +21,33 @@ fn main() -> glib::ExitCode {
         .expect("build tokio runtime");
     let _guard = runtime.enter();
 
+    // Strip `--first-run` from argv before GTK's own option parser sees it;
+    // GTK rejects unknown long options. Phase 10 fills the body of
+    // firstrun::run; the prep lane only wires the entry point.
+    let raw_args: Vec<String> = std::env::args().collect();
+    let first_run = raw_args.iter().any(|a| a == "--first-run");
+    let gtk_args: Vec<String> = raw_args
+        .iter()
+        .filter(|a| a.as_str() != "--first-run")
+        .cloned()
+        .collect();
+
     let app = adw::Application::builder().application_id(APP_ID).build();
-    app.connect_activate(|app| {
+    app.connect_activate(move |app| {
         let win = adw::ApplicationWindow::builder()
             .application(app)
             .default_width(720)
             .default_height(540)
             .title("Authentication")
             .build();
+        let toast_overlay = adw::ToastOverlay::new();
+        let ctx = app_context::AppContext::new(win.clone().upcast(), toast_overlay.clone());
+
+        if first_run {
+            firstrun::run(ctx);
+            return;
+        }
+
         let header = adw::HeaderBar::new();
         let stack = adw::ViewStack::new();
         let switcher = adw::ViewSwitcher::builder()
@@ -36,8 +56,6 @@ fn main() -> glib::ExitCode {
             .build();
         header.set_title_widget(Some(&switcher));
 
-        let toast_overlay = adw::ToastOverlay::new();
-        let ctx = app_context::AppContext::new(win.clone().upcast(), toast_overlay.clone());
         let keys = keys_page::KeysPage::new(ctx.clone());
         stack.add_titled(&keys.root, Some("keys"), "Keys");
 
@@ -52,5 +70,5 @@ fn main() -> glib::ExitCode {
         win.set_content(Some(&toast_overlay));
         win.present();
     });
-    app.run()
+    app.run_with_args(&gtk_args)
 }