Merge PR #73: feat(antigravity): 添加 Antigravity (Cloud AI Companion) 平台支持

新增功能:
- Antigravity OAuth 授权流程支持
- Claude → Gemini 协议转换（Claude API 请求自动转换为 Gemini 格式）
- 配额刷新和状态显示
- 混合调度功能，支持 Anthropic 和 Antigravity 账户混合使用
- /antigravity 专用路由，支持仅使用 Antigravity 账户
- 前端 Antigravity 服务商标识和账户管理功能

冲突解决:
- CreateAccountModal.vue: 合并 data-tour 属性和 mixed-scheduling 属性
- EditAccountModal.vue: 合并 data-tour 属性和 mixed-scheduling 属性

代码质量改进:
- 修复 antigravity 类型文件的 gofmt 格式问题（struct 字段对齐、interface{} → any）
- 移除 .golangci.yml 中的 gofmt 排除规则
- 修复测试文件的格式问题

frontend/src/composables/useAntigravityOAuth.ts

@@ -0,0 +1,115 @@
+import { ref } from 'vue'
+import { useI18n } from 'vue-i18n'
+import { useAppStore } from '@/stores/app'
+import { adminAPI } from '@/api/admin'
+import type { AntigravityTokenInfo } from '@/api/admin/antigravity'
+
+export function useAntigravityOAuth() {
+  const appStore = useAppStore()
+  const { t } = useI18n()
+
+  const authUrl = ref('')
+  const sessionId = ref('')
+  const state = ref('')
+  const loading = ref(false)
+  const error = ref('')
+
+  const resetState = () => {
+    authUrl.value = ''
+    sessionId.value = ''
+    state.value = ''
+    loading.value = false
+    error.value = ''
+  }
+
+  const generateAuthUrl = async (proxyId: number | null | undefined): Promise<boolean> => {
+    loading.value = true
+    authUrl.value = ''
+    sessionId.value = ''
+    state.value = ''
+    error.value = ''
+
+    try {
+      const payload: Record<string, unknown> = {}
+      if (proxyId) payload.proxy_id = proxyId
+
+      const response = await adminAPI.antigravity.generateAuthUrl(payload as any)
+      authUrl.value = response.auth_url
+      sessionId.value = response.session_id
+      state.value = response.state
+      return true
+    } catch (err: any) {
+      error.value =
+        err.response?.data?.detail || t('admin.accounts.oauth.antigravity.failedToGenerateUrl')
+      appStore.showError(error.value)
+      return false
+    } finally {
+      loading.value = false
+    }
+  }
+
+  const exchangeAuthCode = async (params: {
+    code: string
+    sessionId: string
+    state: string
+    proxyId?: number | null
+  }): Promise<AntigravityTokenInfo | null> => {
+    const code = params.code?.trim()
+    if (!code || !params.sessionId || !params.state) {
+      error.value = t('admin.accounts.oauth.antigravity.missingExchangeParams')
+      return null
+    }
+
+    loading.value = true
+    error.value = ''
+
+    try {
+      const payload: Record<string, unknown> = {
+        session_id: params.sessionId,
+        state: params.state,
+        code
+      }
+      if (params.proxyId) payload.proxy_id = params.proxyId
+
+      const tokenInfo = await adminAPI.antigravity.exchangeCode(payload as any)
+      return tokenInfo as AntigravityTokenInfo
+    } catch (err: any) {
+      error.value =
+        err.response?.data?.detail || t('admin.accounts.oauth.antigravity.failedToExchangeCode')
+      appStore.showError(error.value)
+      return null
+    } finally {
+      loading.value = false
+    }
+  }
+
+  const buildCredentials = (tokenInfo: AntigravityTokenInfo): Record<string, unknown> => {
+    let expiresAt: string | undefined
+    if (typeof tokenInfo.expires_at === 'number' && Number.isFinite(tokenInfo.expires_at)) {
+      expiresAt = Math.floor(tokenInfo.expires_at).toString()
+    } else if (typeof tokenInfo.expires_at === 'string' && tokenInfo.expires_at.trim()) {
+      expiresAt = tokenInfo.expires_at.trim()
+    }
+
+    return {
+      access_token: tokenInfo.access_token,
+      refresh_token: tokenInfo.refresh_token,
+      token_type: tokenInfo.token_type,
+      expires_at: expiresAt,
+      project_id: tokenInfo.project_id,
+      email: tokenInfo.email
+    }
+  }
+
+  return {
+    authUrl,
+    sessionId,
+    state,
+    loading,
+    error,
+    resetState,
+    generateAuthUrl,
+    exchangeAuthCode,
+    buildCredentials
+  }
+}