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/Tests/Library/Cases/Public/Controllers/RootControllerTests.swift

@@ -11,9 +11,13 @@ struct RootControllerTests {
     // MARK: Constants
     
     private let app: Application = .init(router: {
-        let router = Router()
-        
-        router.addRoutes(RootController<BasicRequestContext>().routes)
+        let router = Router(context: WebsiteRequestContext.self)
+
+        router.addMiddleware {
+            LocalizationMiddleware()
+        }
+
+        router.addRoutes(RootController<WebsiteRequestContext>().routes)
         
         return router
     }())
@@ -31,6 +35,8 @@ struct RootControllerTests {
 
                 #expect(response.status == .ok)
                 #expect(response.headers[.contentType] == "text/html; charset=utf-8")
+                #expect(response.headers[.contentLanguage] == "en")
+                #expect(response.headers[.vary] == "Accept-Language")
                 #expect(body.contains("Hello world!"))
             }
         }