fix: prevent modal header jumping when toggling API key field

Reserve fixed height for API key input container and use visibility/opacity
for show/hide instead of conditional rendering to maintain consistent modal
height when selecting presets

src/components/AddProviderModal.css

@@ -140,6 +140,18 @@
   font-weight: 500;
 }
 
+/* API Key 输入框容器 - 预留空间避免抖动 */
+.form-group.api-key-group {
+  min-height: 88px; /* 固定高度：label + input + 间距 */
+  transition: opacity 0.2s ease;
+}
+
+.form-group.api-key-group.hidden {
+  opacity: 0;
+  visibility: hidden;
+  pointer-events: none;
+}
+
 .form-group input,
 .form-group textarea {
   width: 100%;

src/components/ProviderForm.tsx

@@ -255,8 +255,7 @@ const ProviderForm: React.FC<ProviderFormProps> = ({
               />
             </div>
 
-            {showApiKey && (
-              <div className="form-group">
+            <div className={`form-group api-key-group ${!showApiKey ? 'hidden' : ''}`}>
               <label htmlFor="apiKey">API Key *</label>
               <input
                 type="text"
@@ -267,7 +266,6 @@ const ProviderForm: React.FC<ProviderFormProps> = ({
                 autoComplete="off"
               />
             </div>
-            )}
 
             <div className="form-group">
               <label htmlFor="websiteUrl">官网地址</label>