feat(frontend): add CSRF injection, rate-limit i18n, and 429 login handling

- handleFetch injects X-XSRF-TOKEN + XSRF-TOKEN cookie on all mutating
  backend API requests (double-submit cookie pattern); generates a fresh
  UUID when no XSRF-TOKEN cookie exists yet
- ErrorCode union gains CSRF_TOKEN_MISSING and TOO_MANY_LOGIN_ATTEMPTS;
  getErrorMessage maps both to i18n keys
- de/en/es messages add error_csrf_token_missing and
  error_too_many_login_attempts translations
- Login action maps HTTP 429 to fail(429, { ..., rateLimited: true });
  page shows a muted clock icon with aria-invalid on rate-limit errors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/en.json

@@ -19,6 +19,8 @@
 	"error_session_expired_explainer": "For security reasons, sessions are automatically ended after 8 hours of inactivity.",
 	"error_unauthorized": "You are not logged in.",
 	"error_forbidden": "You do not have permission for this action.",
+	"error_csrf_token_missing": "Session error. Please reload the page.",
+	"error_too_many_login_attempts": "Too many login attempts. Please try again later.",
 	"error_validation_error": "The input is invalid.",
 	"error_internal_error": "An unexpected error occurred.",
 	"nav_documents": "Documents",