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/tailwind.config.ts

@@ -9,6 +9,8 @@ const config: Config = {
   theme: {
     extend: {
       fontFamily: {
+        sans: ['var(--font-inter)', 'system-ui', 'sans-serif'],
+        inter: ['var(--font-inter)', 'system-ui', 'sans-serif'],
         peyda: ['var(--font-peyda)', 'system-ui', 'sans-serif'],
       },
       colors: {