feat: add "remember me for 30 days" to the login screen

Default stays sessionStorage (signed out on tab close). Checking the box
instead stores the credential in localStorage with an explicit 30-day
expiry checked on every page load, so staying logged in has a hard ceiling
rather than lasting forever.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01855PiW5Wd8n4aRW4n9kByY

frontend/src/lib/i18n.ts

@@ -46,6 +46,7 @@ const ru = {
   "auth.subtitle": "Личные финансы",
   "auth.usernameLabel": "Логин",
   "auth.passwordLabel": "Пароль",
+  "auth.rememberMe": "Запомнить меня на 30 дней",
   "auth.submitButton": "Войти",
   "auth.submitting": "Вход…",
   "auth.errorInvalidCredentials": "Неверный логин или пароль",
@@ -453,6 +454,7 @@ const en: Record<keyof typeof ru, string> = {
   "auth.subtitle": "Personal finance",
   "auth.usernameLabel": "Username",
   "auth.passwordLabel": "Password",
+  "auth.rememberMe": "Remember me for 30 days",
   "auth.submitButton": "Sign in",
   "auth.submitting": "Signing in…",
   "auth.errorInvalidCredentials": "Incorrect username or password",