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/app/composables/useLangSwitch.ts

@@ -0,0 +1,16 @@
+/**
+ * Shared per-navigation state for the language-switch button in the layout.
+ * Each page sets this on setup: `available` is false when there is no
+ * equivalent page in the other language for the current content (e.g. an
+ * untranslated article, or the Italian-only corralito page) — in that case
+ * `fallback` is where the switch should go instead (typically the other
+ * language's blog index).
+ */
+interface LangSwitchState {
+  available: boolean
+  fallback: string
+}
+
+export function useLangSwitchState() {
+  return useState<LangSwitchState>('lang-switch', () => ({ available: true, fallback: '/en' }))
+}