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/views/auth/OAuthCallbackView.vue

@@ -0,0 +1,87 @@
+<template>
+  <div class="min-h-screen bg-gray-50 px-4 py-10 dark:bg-dark-900">
+    <div class="mx-auto max-w-2xl">
+      <div class="card p-6">
+        <h1 class="text-lg font-semibold text-gray-900 dark:text-white">OAuth Callback</h1>
+        <p class="mt-2 text-sm text-gray-600 dark:text-gray-400">
+          Copy the <code>code</code> (and <code>state</code> if needed) back to the admin
+          authorization flow.
+        </p>
+
+        <div class="mt-6 space-y-4">
+          <div>
+            <label class="input-label">Code</label>
+            <div class="flex gap-2">
+              <input class="input flex-1 font-mono text-sm" :value="code" readonly />
+              <button class="btn btn-secondary" type="button" :disabled="!code" @click="copy(code)">
+                Copy
+              </button>
+            </div>
+          </div>
+
+          <div>
+            <label class="input-label">State</label>
+            <div class="flex gap-2">
+              <input class="input flex-1 font-mono text-sm" :value="state" readonly />
+              <button
+                class="btn btn-secondary"
+                type="button"
+                :disabled="!state"
+                @click="copy(state)"
+              >
+                Copy
+              </button>
+            </div>
+          </div>
+
+          <div>
+            <label class="input-label">Full URL</label>
+            <div class="flex gap-2">
+              <input class="input flex-1 font-mono text-xs" :value="fullUrl" readonly />
+              <button
+                class="btn btn-secondary"
+                type="button"
+                :disabled="!fullUrl"
+                @click="copy(fullUrl)"
+              >
+                Copy
+              </button>
+            </div>
+          </div>
+
+          <div
+            v-if="error"
+            class="rounded-lg border border-red-200 bg-red-50 p-3 dark:border-red-700 dark:bg-red-900/30"
+          >
+            <p class="text-sm text-red-600 dark:text-red-400">{{ error }}</p>
+          </div>
+        </div>
+      </div>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { computed } from 'vue'
+import { useRoute } from 'vue-router'
+import { useClipboard } from '@/composables/useClipboard'
+
+const route = useRoute()
+const { copyToClipboard } = useClipboard()
+
+const code = computed(() => (route.query.code as string) || '')
+const state = computed(() => (route.query.state as string) || '')
+const error = computed(
+  () => (route.query.error as string) || (route.query.error_description as string) || ''
+)
+
+const fullUrl = computed(() => {
+  if (typeof window === 'undefined') return ''
+  return window.location.href
+})
+
+const copy = (value: string) => {
+  if (!value) return
+  copyToClipboard(value, 'Copied')
+}
+</script>