✨ Añade soporte para localización y traducción

- Incluye recursos Fluent básicos y pruebas asociadas.
- Nueva variable de configuración global para definir el idioma
  predeterminado.

tests/locale.rs

@@ -0,0 +1,58 @@
+use pagetop::prelude::*;
+
+#[pagetop::test]
+async fn literal_text() {
+    let _app = service::test::init_service(Application::new().test()).await;
+
+    let l10n = L10n::n("© 2025 PageTop");
+    assert_eq!(l10n.get(), Some("© 2025 PageTop".to_string()));
+}
+
+#[pagetop::test]
+async fn translation_without_args() {
+    let _app = service::test::init_service(Application::new().test()).await;
+
+    let l10n = L10n::l("test-hello-world");
+    let translation = l10n.using(LangMatch::langid_or_fallback("es-ES"));
+    assert_eq!(translation, Some("¡Hola mundo!".to_string()));
+}
+
+#[pagetop::test]
+async fn translation_with_args() {
+    let _app = service::test::init_service(Application::new().test()).await;
+
+    let l10n = L10n::l("test-hello-user").with_arg("userName", "Manuel");
+    let translation = l10n.using(LangMatch::langid_or_fallback("es-ES"));
+    assert_eq!(translation, Some("¡Hola, Manuel!".to_string()));
+}
+
+#[pagetop::test]
+async fn translation_with_plural_and_select() {
+    let _app = service::test::init_service(Application::new().test()).await;
+
+    let l10n = L10n::l("test-shared-photos").with_args(vec![
+        ("userName", "Roberto"),
+        ("photoCount", "3"),
+        ("userGender", "male"),
+    ]);
+    let translation = l10n.using(LangMatch::langid_or_fallback("es-ES")).unwrap();
+    assert!(translation.contains("añadido 3 nuevas fotos de él"));
+}
+
+#[pagetop::test]
+async fn check_fallback_language() {
+    let _app = service::test::init_service(Application::new().test()).await;
+
+    let l10n = L10n::l("test-hello-world");
+    let translation = l10n.using(LangMatch::langid_or_fallback("xx-YY")); // Fallback a "en-US".
+    assert_eq!(translation, Some("Hello world!".to_string()));
+}
+
+#[pagetop::test]
+async fn check_unknown_key() {
+    let _app = service::test::init_service(Application::new().test()).await;
+
+    let l10n = L10n::l("non-existent-key");
+    let translation = l10n.using(LangMatch::langid_or_fallback("en-US"));
+    assert_eq!(translation, None);
+}