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/handler/dto/settings.go

@@ -81,6 +81,9 @@ type SystemSettings struct {
 
 	// 分组隔离
 	AllowUngroupedKeyScheduling bool `json:"allow_ungrouped_key_scheduling"`
+
+	// Backend Mode
+	BackendModeEnabled bool `json:"backend_mode_enabled"`
 }
 
 type DefaultSubscriptionSetting struct {
@@ -111,6 +114,7 @@ type PublicSettings struct {
 	CustomMenuItems                  []CustomMenuItem `json:"custom_menu_items"`
 	LinuxDoOAuthEnabled              bool             `json:"linuxdo_oauth_enabled"`
 	SoraClientEnabled                bool             `json:"sora_client_enabled"`
+	BackendModeEnabled               bool             `json:"backend_mode_enabled"`
 	Version                          string           `json:"version"`
 }