fix(subagent): 实验开关改为滑动开关，修复开启态因 bg-primary 未定义而隐形

- Toggle 开启态改用已定义的 bg-blue-600（tailwind 配置无 primary 背景色，
  导致开启态滑轨透明、白色滑块在白底上不可见）
- 子代理开关移至模型下拉之后，统一使用滑动开关

src/components/ui/controls.tsx

@@ -42,6 +42,39 @@ export function Checkbox({
   );
 }
 
+/** Pill-style toggle switch with a sliding knob. */
+export function Toggle({
+  checked,
+  disabled,
+  onChange,
+  ariaLabel,
+}: {
+  checked: boolean;
+  disabled?: boolean;
+  onChange: (checked: boolean) => void;
+  ariaLabel?: string;
+}) {
+  return (
+    <button
+      type="button"
+      role="switch"
+      aria-checked={checked}
+      aria-label={ariaLabel}
+      disabled={disabled}
+      onClick={() => onChange(!checked)}
+      className={`relative h-5 w-9 shrink-0 rounded-full transition-colors duration-200 focus:ring-2 focus:ring-blue-500 focus:outline-none ${
+        checked ? "bg-blue-600" : "bg-border"
+      } ${disabled ? "cursor-not-allowed opacity-50" : ""}`}
+    >
+      <span
+        className={`absolute left-0.5 top-0.5 h-4 w-4 rounded-full bg-white shadow-sm transition-transform duration-200 ${
+          checked ? "translate-x-4" : "translate-x-0"
+        }`}
+      />
+    </button>
+  );
+}
+
 export function NumberField({
   label,
   value,