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/Responses/CachedHTMLResponse.swift

@@ -1,12 +1,16 @@
 import Elementary
+import HTTPTypes
 import Hummingbird
 import NIOCore
 
 /// A pre-rendered HTTP response for a fully static HTML page.
 ///
 /// The document is rendered to bytes once, at initialization, and every ``response()`` reuses those
-/// bytes instead of re-rendering. This suits pages whose markup never changes between requests — the
-/// landing page and the not-found page — avoiding a per-request Elementary render on hot paths.
+/// bytes — along with a fixed status and precomputed headers — instead of re-rendering. This suits
+/// pages whose markup never changes between requests, such as the landing page and the not-found
+/// page, avoiding a per-request Elementary render on hot paths.
+///
+/// ``LocalizedHTMLResponses`` builds on this type, caching one instance per supported language.
 ///
 /// The body is written as an unsized stream (no `Content-Length`), mirroring `HTMLResponse`, so the
 /// response-compression middleware downstream treats it exactly as it would a freshly rendered page.
@@ -14,23 +18,37 @@ struct CachedHTMLResponse: Sendable {
 
     // MARK: Properties
 
-    /// The status applied to every response.
-    private let status: HTTPResponse.Status
     /// The page rendered to bytes once.
     private let buffer: ByteBuffer
+    /// The headers applied to every response, precomputed once.
+    private let headers: HTTPFields
+    /// The status applied to every response.
+    private let status: HTTPResponse.Status
 
     // MARK: Initializers
 
     /// Renders the given document to bytes once.
     /// - Parameters:
     ///   - status: the status applied to every response. Defaults to `.ok`.
+    ///   - additionalHeaders: extra headers merged onto every response, alongside the content type.
+    ///     Used to carry per-language signals such as `Content-Language` and `Vary`.
     ///   - document: the static HTML document to render and cache.
     init(
         status: HTTPResponse.Status = .ok,
-        _ document: some HTMLDocument
+        additionalHeaders: HTTPFields = [:],
+        document: some HTMLDocument
     ) {
+        var headers: HTTPFields = [
+            .contentType: "text/html; charset=utf-8"
+        ]
+
+        for field in additionalHeaders {
+            headers[field.name] = field.value
+        }
+
         self.status = status
-        self.buffer = ByteBuffer(string: document.render())
+        self.headers = headers
+        self.buffer = .init(string: document.render())
     }
 
     // MARK: Methods
@@ -43,7 +61,7 @@ struct CachedHTMLResponse: Sendable {
     func response() -> Response {
         Response(
             status: status,
-            headers: [.contentType: "text/html; charset=utf-8"],
+            headers: headers,
             body: .init { [buffer] writer in
                 try await writer.write(buffer)
                 try await writer.finish(nil)