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/utils/format.ts

@@ -1,13 +1,17 @@
+import type { Locale } from './locale'
+
+const INTL_LOCALE: Record<Locale, string> = { it: 'it-IT', en: 'en-GB' }
+
 /** Human-readable date for display; returns an empty string when unset. */
-export function formatDate(value: string | null | undefined): string {
+export function formatDate(value: string | null | undefined, locale: Locale = 'it'): string {
   if (!value) return ''
-  return new Intl.DateTimeFormat('it-IT', { dateStyle: 'long' }).format(new Date(value))
+  return new Intl.DateTimeFormat(INTL_LOCALE[locale], { dateStyle: 'long' }).format(new Date(value))
 }
 
 /** Date and time of publication, shown in the article byline. */
-export function formatDateTime(value: string | null | undefined): string {
+export function formatDateTime(value: string | null | undefined, locale: Locale = 'it'): string {
   if (!value) return ''
-  return new Intl.DateTimeFormat('it-IT', { dateStyle: 'long', timeStyle: 'short' })
+  return new Intl.DateTimeFormat(INTL_LOCALE[locale], { dateStyle: 'long', timeStyle: 'short' })
     .format(new Date(value))
 }