feat(sites): site requests follow browser UA, smooth UA switch, CF cookies browser-only (BR-4)

- every outbound site request (adapter list/search/detail/download/health,
  RSS, login, user data) carries the account UA via SourcesRuntime.siteFetch;
  no more Deno default UA
- priority: account manual > pending > global setting > browser identity
  (fallback: same-shape Windows Firefox UA); settings UA default is empty
- migration 0056: existing cookie/password accounts keep their old UA as
  ua_pending until re-login or browser cookie write-back switches cookie+UA
  in one transaction; browser_provider.identity caches the browser identity
- cf_clearance/__cf_bm stripped on browser write-back (names kept only)
- browser sessions get per-site egress (same proxy as HTTP, or direct)
- site account page shows UA source; raw UA in tech info

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/contracts/src/settings.ts

@@ -254,8 +254,15 @@ export const MetadataSettings = z.object({
 });
 export type MetadataSettings = z.infer<typeof MetadataSettings>;
 
-/** 固定桌面 Chrome UA(账号未设 UA 时使用) */
-export const DEFAULT_SITE_USER_AGENT =
+/**
+ * 读不到浏览器身份(没有浏览器服务 / 旧镜像 / 暂时连不上)时站点请求用的 UA:与浏览器身份同型的 Windows Firefox
+ * (camofox 1.17.0 = Camoufox 152;升级浏览器镜像时同步改这里,BR-4)。
+ */
+export const FALLBACK_SITE_USER_AGENT =
+  "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:152.0) Gecko/20100101 Firefox/152.0";
+
+/** BR-4 之前的缺省 UA(Windows Chrome 140):迁移时据此把存量账号标"待切换",设置里的旧缺省值清空改为跟随浏览器 */
+export const LEGACY_SITE_USER_AGENT =
   "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/140.0.0.0 Safari/537.36";
 
 /** 站点默认(site-center §8):站点行的限流 / 快照周期为空时继承;站点和凭证在站点页管理 */
@@ -268,8 +275,10 @@ export const SitesSettings = z.object({
     .int()
     .min(300)
     .max(7 * 24 * 3600),
-  /** 账号未设 UA 时使用 */
-  userAgent: z.string().min(1).max(500),
+  /**
+   * 全局 UA 覆盖:空串 = 跟随浏览器(缺省,BR-4);填了则账号没有手动 UA 时都用它(浏览器仍是自己的 UA,两端会不一致)
+   */
+  userAgent: z.string().trim().max(500),
   /** 站点开"使用代理"时生效;null = 不用代理 */
   proxyUrl: z.url().nullable(),
   /** 用户数据做种分页上限 */
@@ -645,7 +654,7 @@ export const DEFAULT_SETTINGS: GlobalSettings = {
   sites: {
     defaultRateLimit: { requestsPerMinute: 20, concurrency: 2 },
     defaultSnapshotIntervalSec: 3600,
-    userAgent: DEFAULT_SITE_USER_AGENT,
+    userAgent: "",
     proxyUrl: null,
     userDataMaxSeedingPages: 10,
     defaultSnapshotMaxPages: 1,