Fix hideLocalLogin toggle persistence and login page UI

- Add hideLocalLogin handler in HandleUpdateSystemSettings() so the
  toggle setting is saved to system.json
- Conditionally hide "or" divider and admin credentials message when
  local login is hidden

Related to #750

frontend-modern/src/components/Login.tsx

@@ -443,16 +443,18 @@ const LoginForm: Component<{
                   {oidcMessage()}
                 </div>
               </Show>
-              <div class="flex items-center gap-3 pt-2">
-                <span class="flex-1 h-px bg-gray-200 dark:bg-gray-700" />
-                <span class="text-xs uppercase tracking-wide text-gray-400 dark:text-gray-500">
-                  or
-                </span>
-                <span class="flex-1 h-px bg-gray-200 dark:bg-gray-700" />
-              </div>
-              <p class="text-xs text-center text-gray-500 dark:text-gray-400">
-                Use your admin credentials to sign in below.
-              </p>
+              <Show when={showLocalLogin()}>
+                <div class="flex items-center gap-3 pt-2">
+                  <span class="flex-1 h-px bg-gray-200 dark:bg-gray-700" />
+                  <span class="text-xs uppercase tracking-wide text-gray-400 dark:text-gray-500">
+                    or
+                  </span>
+                  <span class="flex-1 h-px bg-gray-200 dark:bg-gray-700" />
+                </div>
+                <p class="text-xs text-center text-gray-500 dark:text-gray-400">
+                  Use your admin credentials to sign in below.
+                </p>
+              </Show>
             </div>
           </Show>
           <Show when={showLocalLogin}>

internal/api/system_settings.go

@@ -590,6 +590,9 @@ func (h *SystemSettingsHandler) HandleUpdateSystemSettings(w http.ResponseWriter
 	if _, ok := rawRequest["allowEmbedding"]; ok {
 		settings.AllowEmbedding = updates.AllowEmbedding
 	}
+	if _, ok := rawRequest["hideLocalLogin"]; ok {
+		settings.HideLocalLogin = updates.HideLocalLogin
+	}
 	if _, ok := rawRequest["backupPollingEnabled"]; ok {
 		settings.BackupPollingEnabled = updates.BackupPollingEnabled
 	}