✨ (auth): Añade idioma y tema preferidos al usuario

- CurrentUser pasa de enum a struct con `id` opcional (anónimo = None) y
  preferencias de idioma, zona horaria y tema validadas en `with_*()`.
- `RequestLocale` tiene en cuenta el idioma preferido del usuario, y
  Context usa su tema si lo tiene (`default_theme()` en otro caso).
- Nuevos componentes `form::SelectLanguage`, `form::SelectTheme` y
  `form::SelectTimezone`, con `Timezone::supported_by_region()` para
  listar y validar las zonas IANA ofrecidas.

tests/component_form_select_theme.rs

@@ -0,0 +1,64 @@
+// With more than one enabled theme. The single-theme case lives in its own test binary, because the
+// theme registry is global to the process.
+
+use pagetop::prelude::*;
+
+struct Aurora;
+
+impl Extension for Aurora {
+    fn name(&self) -> Lc {
+        Lc::n("Aurora")
+    }
+
+    fn theme(&self) -> Option<ThemeRef> {
+        Some(&Aurora)
+    }
+}
+
+impl Theme for Aurora {}
+
+async fn setup() {
+    let _ = Application::prepare(&Aurora).await;
+}
+
+#[pagetop::test]
+async fn themes_are_sorted_by_name_and_the_selected_one_is_marked() {
+    setup().await;
+    let mut field = form::SelectTheme::new()
+        .with_name("theme")
+        .with_selected("Basic");
+    let html = field.render(&mut Context::default()).await.into_string();
+
+    let aurora = html.find(r#"<option value="Aurora""#).expect("Aurora");
+    let basic = html
+        .find(r#"<option value="Basic" selected>"#)
+        .expect("Basic");
+    assert!(aurora < basic);
+    assert!(!html.contains("disabled"));
+}
+
+#[pagetop::test]
+async fn optional_field_offers_the_site_theme_first() {
+    setup().await;
+    let mut field = form::SelectTheme::new();
+    let html = field.render(&mut Context::default()).await.into_string();
+
+    let site = html.find(r#"<option value="" selected>Use the site theme: "#);
+    let first_theme = html.find(r#"<option value="Aurora""#);
+    assert!(site.is_some());
+    assert!(site < first_theme);
+}
+
+#[pagetop::test]
+async fn required_field_asks_to_choose_only_when_nothing_valid_is_selected() {
+    setup().await;
+    let mut empty = form::SelectTheme::new().with_required(true);
+    let html = empty.render(&mut Context::default()).await.into_string();
+    assert!(html.contains(r#"<option value="" selected>Choose a theme...</option>"#));
+
+    let mut chosen = form::SelectTheme::new()
+        .with_required(true)
+        .with_selected("Aurora");
+    let html = chosen.render(&mut Context::default()).await.into_string();
+    assert!(!html.contains(r#"value="""#));
+}