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('')
   })

tests/unit/queries.utils.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest'
-import { PAGE_SIZE, pageParam, pagination, quote } from '../../frontend/server/utils/queries'
+import { PAGE_SIZE, langParam, pageParam, pagination, quote } from '../../frontend/server/utils/queries'
 
 describe('pagination', () => {
   it('defaults to PAGE_SIZE', () => {
@@ -25,6 +25,16 @@ describe('pageParam', () => {
   )
 })
 
+describe('langParam', () => {
+  it('accepts "en"', () => {
+    expect(langParam('en')).toBe('en')
+  })
+
+  it.each([undefined, null, '', 'it', 'fr', 123])('falls back to "it" for %p', (value) => {
+    expect(langParam(value)).toBe('it')
+  })
+})
+
 describe('quote', () => {
   it('wraps a plain value in double quotes', () => {
     expect(quote('economia')).toBe('"economia"')