Add i18n foundation (fa-IR/en-US) and localize landing + auth.

Introduce path-prefixed locale routing under app/[lang] with a middleware
that detects locale from cookie/Accept-Language (default fa-IR) and
redirects. Add fa-IR (source of truth) and en-US dictionaries, a server
getDictionary, a client I18nProvider/useT, locale-aware Link + router
helpers, and a language switcher. The root [lang] layout sets html
lang/dir and the per-locale font (Peyda for fa, Inter for en).

Landing sections and the login/register/auth shell now read all copy from
the dictionaries; dashboard localization follows in a later commit.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/middleware.ts

@@ -0,0 +1,53 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { locales, defaultLocale, isLocale, LOCALE_COOKIE, type Locale } from '@/i18n/config';
+
+// Parse the Accept-Language header into an ordered list of base language codes.
+function parseAcceptLanguage(header: string | null): string[] {
+  if (!header) return [];
+  return header
+    .split(',')
+    .map((part) => {
+      const [tag, q] = part.trim().split(';q=');
+      return { tag: tag.trim().toLowerCase(), q: q ? parseFloat(q) : 1 };
+    })
+    .sort((a, b) => b.q - a.q)
+    .map((x) => x.tag);
+}
+
+function detectLocale(request: NextRequest): Locale {
+  // 1. Explicit choice via cookie
+  const cookieLocale = request.cookies.get(LOCALE_COOKIE)?.value;
+  if (isLocale(cookieLocale)) return cookieLocale;
+
+  // 2. Browser preference via Accept-Language
+  const accepted = parseAcceptLanguage(request.headers.get('accept-language'));
+  for (const tag of accepted) {
+    const exact = locales.find((l) => l.toLowerCase() === tag);
+    if (exact) return exact;
+    const base = locales.find((l) => l.toLowerCase().split('-')[0] === tag.split('-')[0]);
+    if (base) return base;
+  }
+
+  // 3. Fallback
+  return defaultLocale;
+}
+
+export function middleware(request: NextRequest) {
+  const { pathname } = request.nextUrl;
+
+  const pathnameHasLocale = locales.some(
+    (locale) => pathname === `/${locale}` || pathname.startsWith(`/${locale}/`),
+  );
+  if (pathnameHasLocale) return;
+
+  const locale = detectLocale(request);
+  request.nextUrl.pathname = `/${locale}${pathname === '/' ? '' : pathname}`;
+  const response = NextResponse.redirect(request.nextUrl);
+  response.cookies.set(LOCALE_COOKIE, locale, { path: '/', maxAge: 60 * 60 * 24 * 365 });
+  return response;
+}
+
+export const config = {
+  // Skip Next internals, the API, and any path containing a dot (static files).
+  matcher: ['/((?!_next|api|.*\\..*).*)'],
+};