Add hand-rolled English support (/en/*) alongside the Italian site

PocketBase gains optional, manually-authored English fields on articles
and categories (same record, same slug), and the frontend serves an /en
counterpart of every dynamic route via thin page wrappers around shared
view components — no i18n library, consistent with the project's existing
minimalism stance for a two-locale site with ~20 UI strings.

An article/category with no translation 404s cleanly on its own /en
detail page and is filtered out of /en listings, and the header's
language-switch link falls back to the English blog index rather than a
dead link; resolving that requires a global route middleware, since the
layout's header renders before the page content in document order and so
can't react to state a page component sets during its own async setup.

frontend/shared/types/blog.ts

@@ -26,6 +26,8 @@ export interface Article extends ArticleSummary {
   summary: string
   /** Byline: the article's authorName field, filled in manually by the editor. */
   author: string | null
+  /** Whether this article has a (manually authored) English translation. */
+  hasTranslation: boolean
 }
 
 export interface Paginated<T> {