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/Package.swift

@@ -4,6 +4,7 @@ import PackageDescription
 
 let package = Package(
     name: "Website",
+    defaultLocalization: "en",
     platforms: [
         .macOS(.v15),
         .iOS(.v18),
@@ -19,6 +20,9 @@ let package = Package(
         )
     ],
     dependencies: [
+        .package(
+            path: "../Packages/Localization"
+        ),
         .package(
             url: "https://github.com/elementary-swift/elementary.git",
             from: "0.6.0"
@@ -67,6 +71,7 @@ let package = Package(
         .target(
             name: "WebsiteCore",
             dependencies: [
+                .byName(name: "Localization"),
                 .product(
                     name: "Configuration",
                     package: "swift-configuration"
@@ -84,7 +89,10 @@ let package = Package(
                     package: "hummingbird-elementary"
                 ),
             ],
-            path: "Sources/Library"
+            path: "Sources/Library",
+            resources: [
+                .process("Catalogs/Localizable.xcstrings")
+            ]
         ),
         .testTarget(
             name: "WebsiteTests",