feat(frontend): 新增 Gemini OAuth 授权流程

- 新增 /admin/gemini API 接口封装（generateAuthUrl, exchangeCode）
- 新增 useGeminiOAuth composable 处理 Gemini OAuth 流程
- 新增 OAuthCallbackView 视图用于接收 OAuth 回调
- 支持 code/state 参数提取和 credentials 构建

frontend/src/api/admin/gemini.ts

@@ -0,0 +1,47 @@
+/**
+ * Admin Gemini API endpoints
+ * Handles Gemini OAuth flows for administrators
+ */
+
+import { apiClient } from '../client'
+
+export interface GeminiAuthUrlResponse {
+  auth_url: string
+  session_id: string
+  state: string
+}
+
+export interface GeminiAuthUrlRequest {
+  redirect_uri: string
+  proxy_id?: number
+}
+
+export interface GeminiExchangeCodeRequest {
+  session_id: string
+  state: string
+  code: string
+  redirect_uri: string
+  proxy_id?: number
+}
+
+export type GeminiTokenInfo = Record<string, unknown>
+
+export async function generateAuthUrl(
+  payload: GeminiAuthUrlRequest
+): Promise<GeminiAuthUrlResponse> {
+  const { data } = await apiClient.post<GeminiAuthUrlResponse>(
+    '/admin/gemini/oauth/auth-url',
+    payload
+  )
+  return data
+}
+
+export async function exchangeCode(payload: GeminiExchangeCodeRequest): Promise<GeminiTokenInfo> {
+  const { data } = await apiClient.post<GeminiTokenInfo>(
+    '/admin/gemini/oauth/exchange-code',
+    payload
+  )
+  return data
+}
+
+export default { generateAuthUrl, exchangeCode }