feat(antigravity): 对齐 AM 客户端标识，前端 OAuth 增加 TOTP 支持

- User-Agent 改为 Electron 浏览器格式，新增 x-client-name/x-client-version/x-vscode-sessionid/x-goog-api-client header
- 移除 MODEL_ALIAS_MAP 别名映射，改为仅剥离 -online 后缀
- thinking 自动注入关键字新增 gemini-3.1-pro
- 新增定时 Antigravity UA 版本刷新任务（每 6 小时 + 启动时）
- 前端 OAuth 设备授权新增 TOTP 验证码生成（otpauth），Region 改为按钮选择
- SSO OIDC 轮询日志 authorization_pending/slow_down 降级为 debug

frontend/src/composables/useTotp.ts

@@ -0,0 +1,79 @@
+import { ref, computed, onBeforeUnmount } from 'vue'
+import { TOTP, Secret } from 'otpauth'
+
+/**
+ * TOTP composable: 给定 secret 持续生成验证码并显示剩余秒数。
+ */
+export function useTotp() {
+  const secret = ref('')
+  const code = ref('')
+  const remaining = ref(0)
+  const error = ref('')
+
+  let timer: ReturnType<typeof setInterval> | null = null
+  let totp: TOTP | null = null
+
+  function parseSecret(raw: string): Secret | null {
+    const cleaned = raw.replace(/[\s-]/g, '')
+    if (!cleaned) return null
+    try {
+      return Secret.fromBase32(cleaned.toUpperCase())
+    } catch {
+      // 尝试 hex
+    }
+    try {
+      return Secret.fromHex(cleaned)
+    } catch {
+      return null
+    }
+  }
+
+  function generate(): void {
+    if (!totp) {
+      code.value = ''
+      remaining.value = 0
+      return
+    }
+    code.value = totp.generate()
+    const epoch = Math.floor(Date.now() / 1000)
+    remaining.value = totp.period - (epoch % totp.period)
+  }
+
+  function start(rawSecret: string): void {
+    stop()
+    error.value = ''
+    secret.value = rawSecret
+
+    const parsed = parseSecret(rawSecret)
+    if (!parsed) {
+      error.value = rawSecret.trim() ? 'Secret 格式无效' : ''
+      return
+    }
+
+    totp = new TOTP({ secret: parsed, digits: 6, period: 30 })
+    generate()
+    timer = setInterval(generate, 1000)
+  }
+
+  function stop(): void {
+    if (timer) {
+      clearInterval(timer)
+      timer = null
+    }
+    totp = null
+    code.value = ''
+    remaining.value = 0
+    error.value = ''
+  }
+
+  onBeforeUnmount(stop)
+
+  return {
+    secret,
+    code: computed(() => code.value),
+    remaining: computed(() => remaining.value),
+    error: computed(() => error.value),
+    start,
+    stop,
+  }
+}