✨ Añade gestión de errores catastróficos

- Los errores controlados (403, 404, fatales) se renderizan con el tema
  activo; un `panic!` no controlado se intercepta con un HTML mínimo y
  autónomo, sin depender del tema ni de los componentes.
- Corrige además que `Context::template()` no seguía los cambios de tema
  hechos con `with_theme()` después de crear la página.
- Renombra `DefaultRegion(s)`/`DefaultTemplate(s)`, añade
  `Theme::admin_template()` y `Page::admin()`, y documenta la creación
  de temas e `InRegion` en `core/theme.rs`.

tests/error_handling.rs

@@ -0,0 +1,54 @@
+use pagetop::prelude::*;
+
+// **< CatchPanicLayer >****************************************************************************
+
+struct PanicExtension;
+
+#[async_trait]
+impl Extension for PanicExtension {
+    fn configure_router(&self, router: Router) -> Router {
+        router.route("/boom", web::get(boom))
+    }
+}
+
+async fn boom() -> Result<Markup, ErrorPage> {
+    panic!("boom")
+}
+
+#[pagetop::test]
+async fn panic_in_handler_returns_minimal_500_page_instead_of_crashing() {
+    let app = web::test::init_router(Application::prepare(&PanicExtension).await.test());
+
+    let req = web::test::TestRequest::get().uri("/boom").to_request();
+    let resp = web::test::send_request(&app, req).await;
+
+    assert_eq!(resp.status(), web::http::StatusCode::INTERNAL_SERVER_ERROR);
+    assert_eq!(
+        resp.headers().get(web::http::header::CONTENT_TYPE).unwrap(),
+        "text/html; charset=utf-8"
+    );
+
+    let body = web::test::read_body_text(resp).await;
+    assert!(body.contains("An unexpected error has occurred"));
+}
+
+// **< ErrorPage::NotFound >************************************************************************
+
+// `EXTENSIONS` es un `OnceLock` global (`core/extension/all.rs`): se inicializa una sola vez por
+// binario de test. Todos los tests de este fichero comparten la misma extensión raíz
+// (`PanicExtension`) para que el orden de ejecución en paralelo no cambie qué rutas quedan
+// registradas.
+#[pagetop::test]
+async fn unknown_route_returns_themed_404_page() {
+    let app = web::test::init_router(Application::prepare(&PanicExtension).await.test());
+
+    let req = web::test::TestRequest::get()
+        .uri("/does-not-exist")
+        .to_request();
+    let resp = web::test::send_request(&app, req).await;
+
+    assert_eq!(resp.status(), web::http::StatusCode::NOT_FOUND);
+
+    let body = web::test::read_body_text(resp).await;
+    assert!(body.contains("<html"));
+}

tests/theme_template.rs

@@ -0,0 +1,83 @@
+use pagetop::prelude::*;
+
+// **< Tema con plantilla propia >******************************************************************
+
+struct MarkerTemplate;
+
+#[async_trait]
+impl Template for MarkerTemplate {
+    async fn render(&self, _cx: &mut Context) -> Markup {
+        html! { "marker-template-output" }
+    }
+}
+
+struct MarkerTheme;
+
+#[async_trait]
+impl Extension for MarkerTheme {
+    fn theme(&self) -> Option<ThemeRef> {
+        Some(&Self)
+    }
+}
+
+#[async_trait]
+impl Theme for MarkerTheme {
+    fn default_template(&self) -> TemplateRef {
+        &MarkerTemplate
+    }
+
+    fn admin_template(&self) -> TemplateRef {
+        &MarkerTemplate
+    }
+}
+
+async fn render_active_template(cx: &mut Context) -> String {
+    let template = cx.template();
+    template.render(cx).await.into_string()
+}
+
+// **< Context::template() sigue al tema activo >***************************************************
+
+#[pagetop::test]
+async fn with_theme_updates_the_effective_template() {
+    // Sin cambiar de tema, la plantilla activa no es la de `MarkerTheme`.
+    let mut cx = Context::new(None);
+    assert_ne!(
+        render_active_template(&mut cx).await,
+        "marker-template-output"
+    );
+
+    // Tras cambiar de tema con `with_theme()`, la plantilla activa pasa a ser la de ese tema, sin
+    // necesidad de llamar a `with_template()` explícitamente.
+    let mut cx = Context::new(None).with_theme(&MarkerTheme);
+    assert_eq!(
+        render_active_template(&mut cx).await,
+        "marker-template-output"
+    );
+}
+
+#[pagetop::test]
+async fn explicit_template_is_not_overridden_by_a_later_with_theme() {
+    // Una plantilla fijada explícitamente con `with_template()` prevalece aunque `with_theme()` se
+    // llame después, en cualquier orden.
+    let mut cx = Context::new(None)
+        .with_template(&MarkerTemplate)
+        .with_theme(&pagetop::base::theme::Basic);
+
+    assert_eq!(
+        render_active_template(&mut cx).await,
+        "marker-template-output"
+    );
+}
+
+// **< Page::admin() sigue al tema activo >*********************************************************
+
+#[pagetop::test]
+async fn page_admin_template_follows_a_later_with_theme() {
+    let request = web::test::TestRequest::get().to_http_request();
+
+    let mut page = Page::admin(request).with_theme(&MarkerTheme);
+    let markup = page.context().template().render(page.context()).await;
+
+    assert_eq!(markup.into_string(), "marker-template-output");
+}