feat: admin can create users manually + disable registration (#85)

Backend:
- POST /api/admin/users — admin-only user creation endpoint
  - username & password required, email optional (auto-generated placeholder)
  - role, quota_bytes, active all configurable
  - creates personal folder automatically
- PUT /api/admin/users/{id}/password — admin password reset
- GET/PUT /api/admin/settings/registration — toggle public registration
  - Supports env var OXICLOUD_DISABLE_REGISTRATION override
  - Blocks POST /api/auth/register when disabled
- AdminCreateUserDto, AdminResetPasswordDto added to settings DTOs
- registration_enabled field added to DashboardStatsDto

Frontend (admin.html):
- 'Create User' button in Users tab with full modal form
  (username, password, email, role, quota)
- 'Reset Password' button per user in actions column
- 'Allow public self-registration' toggle in Dashboard > System
  with warning banner when disabled

Closes #85

src/interfaces/api/handlers/auth_handler.rs

@@ -65,6 +65,17 @@ async fn register(
             "PasswordRegistrationDisabled",
         ));
     }
+
+    // Check if public registration has been disabled by the admin
+    if let Some(admin_svc) = state.admin_settings_service.as_ref() {
+        if !admin_svc.get_registration_enabled().await {
+            return Err(AppError::new(
+                StatusCode::FORBIDDEN,
+                "Public registration has been disabled by the administrator.",
+                "RegistrationDisabled",
+            ));
+        }
+    }
     
     // Registration logic (admin detection, fresh-install handling, duplicate
     // checks) is all inside the service layer. Call it directly.