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/i18n/dictionaries.ts

@@ -0,0 +1,11 @@
+import type { Locale } from './config';
+import type { Dictionary } from './dictionaries/fa';
+
+const loaders: Record<Locale, () => Promise<Dictionary>> = {
+  'fa-IR': () => import('./dictionaries/fa').then((m) => m.default),
+  'en-US': () => import('./dictionaries/en').then((m) => m.default),
+};
+
+export const getDictionary = async (locale: Locale): Promise<Dictionary> => loaders[locale]();
+
+export type { Dictionary };