Localization support for the Website service (#10)

This PR contains the work done to introduce server-side localization support to the **Website** service so the landing and error pages are served in the visitor's negotiated language, backed by a new reusable Localization package.

To provide further details about the work:

* Created the _Localization_ package — a bundle-bound `Localize` method and a `LanguageList` type.
* Language negotiation — `NegotiateLanguage` method picks the best supported language from the request's _Accept-Language_ header (falling back to the default); the `LocalizationMiddleware` middleware resolves it per request and stores it on a new `LocalizedRequestContext` / `WebsiteRequestContext` context.
* Localized responses — `LocalizedHTMLCollectionResponse` pre-renders each page once per language and caches the bytes (with `Content-Language` + `Vary: Accept-Language`), reused by the  `RootController` controller and `NotFoundMiddleware` middleware.
* The `CachedHTMLResponse` response gained custom-header support.
* Localized pages — the `IndexPage` and `ErrorPage` pages now resolve their strings via `Localize` method;
* Added `Localizable.xcstrings` catalogs.
* Wired the `LocalizationMiddleware` middlewaer into the router.

Reviewed-on: rock-n-code/loud-amsterdam#10
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Sources/Library/Internal/Pages/IndexPage.swift

@@ -1,13 +1,36 @@
 import Elementary
+import Foundation
+import Localization
 
-/// The website's landing page.
+/// The website's landing page, with its text localized to a given locale.
 struct IndexPage: HTMLDocument, Sendable {
 
+    // MARK: Properties
+
+    /// The locale the page content is localized to.
+    private let locale: Locale
+
+    /// Resolves the page's text from the bundled String Catalog for the page's ``locale``.
+    private let localize: Localize
+
+    // MARK: Initializers
+
+    /// Creates a landing page localized to the given locale.
+    /// - Parameter locale: the locale the page content is localized to.
+    init(
+        locale: Locale
+    ) {
+        self.locale = locale
+        self.localize = .init(bundle: .module)
+    }
+
     // MARK: Document
 
-    /// The page's content.
+    /// The page's content: a localized greeting followed by the app script.
     var body: some HTML {
-        p { "Hello world! This is HTML5 Boilerplate." }
+        p {
+            localize("index.greeting", locale: locale)
+        }
         script(.src("/js/app.js")) {}
     }
 
@@ -52,14 +75,15 @@ struct IndexPage: HTMLDocument, Sendable {
         )
     }
 
-    /// The document language.
+    /// The document language, derived from the page's locale and falling back to the default language.
     var lang: String {
-        "en"
+        locale.language.languageCode?.identifier
+            ?? LanguageList(bundle: .module).default
     }
 
-    /// The document title.
+    /// The localized document title.
     var title: String {
-        "Index page"
+        localize("index.title", locale: locale)
     }
 
 }