✨ (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_timezone.rs

@@ -0,0 +1,63 @@
+use pagetop::prelude::*;
+
+#[pagetop::test]
+async fn groups_zones_by_region_and_marks_the_selected_one() {
+    let mut field = form::SelectTimezone::new()
+        .with_name("timezone")
+        .with_selected("Europe/Madrid");
+    let html = field.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains(r#"<optgroup label="Europe">"#));
+    assert!(html.contains(r#"<option value="Europe/Madrid" selected>"#));
+    assert!(html.contains(r#"value="Etc/UTC""#));
+}
+
+#[pagetop::test]
+async fn region_labels_are_translated() {
+    let mut field = form::SelectTimezone::new();
+    let mut cx = Context::default().with_langid(&Locale::resolve("es-ES"));
+    let html = field.render(&mut cx).await.into_string();
+
+    assert!(html.contains(r#"<optgroup label="Europa">"#));
+    assert!(html.contains(r#"<optgroup label="Otras">"#));
+}
+
+#[pagetop::test]
+async fn legacy_and_fixed_offset_zones_are_not_offered() {
+    let mut field = form::SelectTimezone::new();
+    let html = field.render(&mut Context::default()).await.into_string();
+
+    assert!(!html.contains(r#"value="US/Eastern""#));
+    assert!(!html.contains(r#"value="Japan""#));
+    assert!(!html.contains(r#"value="Etc/GMT+1""#));
+}
+
+#[pagetop::test]
+async fn optional_field_offers_the_site_time_zone_first() {
+    let mut field = form::SelectTimezone::new();
+    let html = field.render(&mut Context::default()).await.into_string();
+
+    let site = html.find(r#"<option value="" selected>Use the site time zone: "#);
+    let first_group = html.find("<optgroup");
+    assert!(site.is_some());
+    assert!(site < first_group);
+}
+
+#[pagetop::test]
+async fn required_field_asks_to_choose_only_when_nothing_valid_is_selected() {
+    let mut empty = form::SelectTimezone::new().with_required(true);
+    let html = empty.render(&mut Context::default()).await.into_string();
+    assert!(html.contains(r#"<option value="" selected>Choose a time zone...</option>"#));
+
+    let mut unknown = form::SelectTimezone::new()
+        .with_required(true)
+        .with_selected("Mars/Olympus");
+    let html = unknown.render(&mut Context::default()).await.into_string();
+    assert!(html.contains(r#"<option value="" selected>Choose a time zone...</option>"#));
+
+    let mut chosen = form::SelectTimezone::new()
+        .with_required(true)
+        .with_selected("Europe/Madrid");
+    let html = chosen.render(&mut Context::default()).await.into_string();
+    assert!(!html.contains(r#"value="""#));
+}