✨ (base): Añade componentes Nav y Navbar

Dropdown incorpora tamaño y estilo de botón configurables
(`button::Size`/`Style`), reutilizados también por Navbar. El tema Basic
sirve `basic.min.css` y añade el JS de inicialización de Navbar.

examples/basic-nav.rs

@@ -0,0 +1,46 @@
+use pagetop::prelude::*;
+
+struct NavDemo;
+
+#[async_trait]
+impl Extension for NavDemo {
+    fn dependencies(&self) -> Vec<ExtensionRef> {
+        vec![&pagetop::base::extension::Welcome]
+    }
+
+    async fn initialize(&self) {
+        let main_nav = Nav::new()
+            .with_item(nav::Item::link(Lc::n("Home"), "/"))
+            .with_item(nav::Item::link_blank(
+                Lc::n("Docs"),
+                "https://docs.rs/pagetop",
+            ))
+            .with_item(nav::Item::dropdown(
+                Dropdown::new()
+                    .with_title(Lc::n("Tools"))
+                    .with_item(dropdown::Item::link(Lc::n("Generator"), "/tools/gen"))
+                    .with_item(dropdown::Item::link(Lc::n("Reports"), "/tools/reports"))
+                    .with_item(dropdown::Item::divider())
+                    .with_item(dropdown::Item::header(Lc::n("Danger zone")))
+                    .with_item(dropdown::Item::button(Lc::n("Reset settings"))),
+            ))
+            .with_item(nav::Item::link_disabled(Lc::n("Disabled"), "#"));
+
+        let user_menu = Dropdown::new()
+            .with_title(Lc::n("Account"))
+            .with_item(dropdown::Item::label(Lc::n("Signed in as demo")))
+            .with_item(dropdown::Item::divider())
+            .with_item(dropdown::Item::link(Lc::n("Profile"), "/profile"))
+            .with_item(dropdown::Item::link(Lc::n("Settings"), "/settings"))
+            .with_item(dropdown::Item::divider())
+            .with_item(dropdown::Item::button(Lc::n("Sign out")));
+
+        InRegion::Global(&CoreRegions::Header).add(main_nav);
+        InRegion::Global(&CoreRegions::Aside).add(user_menu);
+    }
+}
+
+#[pagetop::main]
+async fn main() -> std::io::Result<()> {
+    Application::prepare(&NavDemo).await.run().await
+}