feat: add Backend Mode toggle to disable user self-service

Add a system-wide "Backend Mode" that disables user self-registration
and self-service while keeping admin panel and API gateway fully
functional. When enabled, only admin can log in; all user-facing
routes return 403.

Backend:
- New setting key `backend_mode_enabled` with atomic cached reads (60s TTL)
- BackendModeUserGuard middleware blocks non-admin authenticated routes
- BackendModeAuthGuard middleware blocks registration/password-reset auth routes
- Login/Login2FA/RefreshToken handlers reject non-admin when enabled
- TokenPairWithUser struct for role-aware token refresh
- 20 unit tests (middleware + service layer)

Frontend:
- Router guards redirect unauthenticated users to /login
- Admin toggle in Settings page
- Login page hides register link and footer in backend mode
- 9 unit tests for router guard logic
- i18n support (en/zh)

27 files changed, 833 insertions(+), 17 deletions(-)

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

backend/internal/service/auth_service.go

@@ -1087,6 +1087,12 @@ type TokenPair struct {
 	ExpiresIn    int    `json:"expires_in"` // Access Token有效期（秒）
 }
 
+// TokenPairWithUser extends TokenPair with user role for backend mode checks
+type TokenPairWithUser struct {
+	TokenPair
+	UserRole string
+}
+
 // GenerateTokenPair 生成Access Token和Refresh Token对
 // familyID: 可选的Token家族ID，用于Token轮转时保持家族关系
 func (s *AuthService) GenerateTokenPair(ctx context.Context, user *User, familyID string) (*TokenPair, error) {
@@ -1168,7 +1174,7 @@ func (s *AuthService) generateRefreshToken(ctx context.Context, user *User, fami
 
 // RefreshTokenPair 使用Refresh Token刷新Token对
 // 实现Token轮转：每次刷新都会生成新的Refresh Token，旧Token立即失效
-func (s *AuthService) RefreshTokenPair(ctx context.Context, refreshToken string) (*TokenPair, error) {
+func (s *AuthService) RefreshTokenPair(ctx context.Context, refreshToken string) (*TokenPairWithUser, error) {
 	// 检查 refreshTokenCache 是否可用
 	if s.refreshTokenCache == nil {
 		return nil, ErrRefreshTokenInvalid
@@ -1233,7 +1239,14 @@ func (s *AuthService) RefreshTokenPair(ctx context.Context, refreshToken string)
 	}
 
 	// 生成新的Token对，保持同一个家族ID
-	return s.GenerateTokenPair(ctx, user, data.FamilyID)
+	pair, err := s.GenerateTokenPair(ctx, user, data.FamilyID)
+	if err != nil {
+		return nil, err
+	}
+	return &TokenPairWithUser{
+		TokenPair: *pair,
+		UserRole:  user.Role,
+	}, nil
 }
 
 // RevokeRefreshToken 撤销单个Refresh Token