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.

tests/unit/format.utils.test.ts

@@ -4,10 +4,14 @@ import { formatDate, formatDateTime, isoDate } from '../../frontend/shared/utils
 const SAMPLE = '2025-01-15T10:30:00.000Z'
 
 describe('formatDate', () => {
-  it('formats a date in long it-IT style', () => {
+  it('formats a date in long it-IT style by default', () => {
     expect(formatDate(SAMPLE)).toBe('15 gennaio 2025')
   })
 
+  it('formats a date in long en-GB style when locale is en', () => {
+    expect(formatDate(SAMPLE, 'en')).toBe('15 January 2025')
+  })
+
   it('returns an empty string when unset', () => {
     expect(formatDate(null)).toBe('')
     expect(formatDate(undefined)).toBe('')
@@ -20,6 +24,11 @@ describe('formatDateTime', () => {
     expect(result).toContain('15 gennaio 2025')
   })
 
+  it('includes both date and time in English', () => {
+    const result = formatDateTime(SAMPLE, 'en')
+    expect(result).toContain('15 January 2025')
+  })
+
   it('returns an empty string when unset', () => {
     expect(formatDateTime(null)).toBe('')
   })