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/components/LanguageSwitch.vue

@@ -0,0 +1,37 @@
+<script setup lang="ts">
+import { STRINGS } from '#shared/utils/i18n'
+import { bareLocalePath, localePath } from '#shared/utils/locale'
+
+const route = useRoute()
+const locale = useLocale()
+const langSwitch = useLangSwitchState()
+
+const target = computed(() => {
+  if (!langSwitch.value.available) return langSwitch.value.fallback
+  const otherLocale = locale.value === 'en' ? 'it' : 'en'
+  return localePath(otherLocale, bareLocalePath(route.path))
+})
+
+const label = computed(() => STRINGS[locale.value].langSwitchLabel)
+</script>
+
+<template>
+  <NuxtLink class="lang-switch" :to="target">{{ label }}</NuxtLink>
+</template>
+
+<style scoped>
+.lang-switch {
+  display: inline-block;
+  font-size: 0.75rem;
+  font-weight: 600;
+  letter-spacing: 0.14em;
+  text-transform: uppercase;
+  text-decoration: none;
+  padding-block: 0.6rem;
+  border-bottom: 1px solid transparent;
+}
+
+.lang-switch:hover {
+  border-bottom-color: var(--color-ink);
+}
+</style>