fix(ai): treat unchanged AI settings as successful save

update_option returns false when values are identical, which made
POST /ai/settings respond 400 with "保存失败" on no-op saves and
masked-key re-saves. Normalize config, accept no-op as success, and
only send a new API key from the admin form when entered.

assets/admin/src/pages/extensions/ExtensionAI.vue

@@ -216,9 +216,17 @@ export default {
     async function saveSettings() {
       saving.value = true;
       try {
-        const payload = { ...form };
-        if (!payload.api_key && status.value.configured) {
-          payload.api_key = '***';
+        const payload = {
+          enabled: form.enabled,
+          provider: form.provider,
+          api_base: form.api_base,
+          model: form.model,
+          max_tokens: form.max_tokens,
+          rate_limit: form.rate_limit,
+        };
+        // Only send api_key when the user entered a new one.
+        if (form.api_key && form.api_key !== '***') {
+          payload.api_key = form.api_key;
         }
         const json = await api.updateAiSettings(payload);
         alert(json.message || '设置已保存');