feat(gui): Policy tab with three master stacks and lockout-violation banner

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

gui/src/main.rs

@@ -8,6 +8,7 @@ mod error;
 mod firstrun;
 mod keys_page;
 mod policy_form;
+mod policy_page;
 
 const APP_ID: &str = "io.dangerousthings.AuthForge";
 
@@ -60,6 +61,9 @@ fn main() -> glib::ExitCode {
         let keys = keys_page::KeysPage::new(ctx.clone());
         stack.add_titled(&keys.root, Some("keys"), "Keys");
 
+        let policy = policy_page::PolicyPage::new(ctx.clone());
+        stack.add_titled(&policy.root, Some("policy"), "Policy");
+
         // ToolbarView is libadwaita v1_4-gated; the project sticks with the
         // simpler Box layout for portability (see commit c6a5e94).
         let layout = gtk::Box::new(gtk::Orientation::Vertical, 0);