feat: branded login screen in front of Basic Auth

The browser's native Basic Auth prompt can't be restyled — it's chrome UI,
not part of the page. Replace it with Aurum's own login screen: nginx keeps
auth_basic on /api/* (the actual protection, unchanged), but drops it from
the SPA shell (just HTML/JS/CSS, no data) so the app can render its own
branded form instead. On submit the app validates the credentials against a
real protected endpoint, then attaches the resulting Authorization header to
every request itself — sessionStorage-backed, so closing the tab signs out.
Instances with Basic Auth left off (the default) never see this screen.

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

frontend/src/lib/i18n.ts

@@ -43,6 +43,14 @@ const ru = {
   "common.perMonth": "/мес",
   "common.allShort": "Всё",
 
+  "auth.subtitle": "Личные финансы",
+  "auth.usernameLabel": "Логин",
+  "auth.passwordLabel": "Пароль",
+  "auth.submitButton": "Войти",
+  "auth.submitting": "Вход…",
+  "auth.errorInvalidCredentials": "Неверный логин или пароль",
+  "auth.errorUnreachable": "Не удалось подключиться к серверу. Проверьте соединение и попробуйте снова.",
+
   "dashboard.errorLoading": "Не удалось загрузить данные дашборда. Проверьте, что backend запущен.",
   "dashboard.statRealIncomeLabel": "Реальный доход",
   "dashboard.statRealIncomeCaption": "зарплаты, возвраты",
@@ -442,6 +450,14 @@ const en: Record<keyof typeof ru, string> = {
   "common.perMonth": "/mo",
   "common.allShort": "All",
 
+  "auth.subtitle": "Personal finance",
+  "auth.usernameLabel": "Username",
+  "auth.passwordLabel": "Password",
+  "auth.submitButton": "Sign in",
+  "auth.submitting": "Signing in…",
+  "auth.errorInvalidCredentials": "Incorrect username or password",
+  "auth.errorUnreachable": "Couldn't reach the server. Check your connection and try again.",
+
   "dashboard.errorLoading": "Failed to load dashboard data. Check that the backend is running.",
   "dashboard.statRealIncomeLabel": "Real income",
   "dashboard.statRealIncomeCaption": "paychecks, refunds",