feat(frontend): 添加 Gemini OAuth 能力查询 API

- 添加 GeminiOAuthCapabilities 类型定义
- 添加 getCapabilities API 函数
- useGeminiOAuth composable 导出 getCapabilities 方法

frontend/src/api/admin/gemini.ts

@@ -11,6 +11,11 @@ export interface GeminiAuthUrlResponse {
   state: string
 }
 
+export interface GeminiOAuthCapabilities {
+  ai_studio_oauth_enabled: boolean
+  required_redirect_uris: string[]
+}
+
 export interface GeminiAuthUrlRequest {
   proxy_id?: number
   project_id?: string
@@ -45,4 +50,9 @@ export async function exchangeCode(payload: GeminiExchangeCodeRequest): Promise<
   return data
 }
 
-export default { generateAuthUrl, exchangeCode }
+export async function getCapabilities(): Promise<GeminiOAuthCapabilities> {
+  const { data } = await apiClient.get<GeminiOAuthCapabilities>('/admin/gemini/oauth/capabilities')
+  return data
+}
+
+export default { generateAuthUrl, exchangeCode, getCapabilities }

frontend/src/composables/useGeminiOAuth.ts

@@ -2,6 +2,7 @@ import { ref } from 'vue'
 import { useI18n } from 'vue-i18n'
 import { useAppStore } from '@/stores/app'
 import { adminAPI } from '@/api/admin'
+import type { GeminiOAuthCapabilities } from '@/api/admin/gemini'
 
 export interface GeminiTokenInfo {
   access_token?: string
@@ -119,6 +120,15 @@ export function useGeminiOAuth() {
     }
   }
 
+  const getCapabilities = async (): Promise<GeminiOAuthCapabilities | null> => {
+    try {
+      return await adminAPI.gemini.getCapabilities()
+    } catch (err: any) {
+      // Capabilities are optional for older servers; don't block the UI.
+      return null
+    }
+  }
+
   return {
     authUrl,
     sessionId,
@@ -128,6 +138,7 @@ export function useGeminiOAuth() {
     resetState,
     generateAuthUrl,
     exchangeAuthCode,
-    buildCredentials
+    buildCredentials,
+    getCapabilities
   }
 }