浏览器免重登：HttpOnly cookie + 服务端 session 表 + 设备名持久化

- 新增 web_sessions 表（sqlc）：refresh_token 以 AES-256-GCM 落盘，密钥取自
  CDROP_SESSION_SECRET（env、不在库内）；cookie 仅存不透明随机串，表 id 存其
  SHA-256，故 DB 单独泄露既换不出可用 cookie、也解不出 token
- /auth/exchange 建 session 并下发 HttpOnly; Secure; SameSite=Lax; Path=/api/auth
  cookie，响应体不再回传 refresh_token
- /auth/refresh 改为 cookie 驱动（7 天滑动失活），同时即开机静默免重登；
  新增 /auth/logout（删 session + 清 cookie）、/auth/device（写设备名入 session）
- device_name 随 session 存，开机 refresh 带回前端，抗 iOS PWA 存储清除；
  setup / 改名时 syncWebDeviceName 推送服务端
- striped per-session 锁串行化同会话并发 refresh，防一次性 refresh_token 被
  花两次而把用户从所有 tab 登出
- 前端 store 移除 refreshToken 字段（长寿命凭据彻底不进 JS）；main.tsx 首屏前
  bootstrapAuth 用 cookie 静默续期，命中直接进已登录态、避免登录页闪现
- config：prod 强制 CDROP_SESSION_SECRET，缺失拒启动
- 桌面端不受影响（自有 loopback flow + Go keyring，不碰这两个端点）

internal/jwtauth/middleware.go

@@ -64,7 +64,7 @@ func (a *Authenticator) Middleware(next http.Handler) http.Handler {
 			return
 		}
 
-		deviceName := sanitizeDeviceName(r.Header.Get("X-Device-Name"))
+		deviceName := SanitizeDeviceName(r.Header.Get("X-Device-Name"))
 		deviceType := normalizeDeviceType(r.Header.Get("X-Device-Type"))
 		if claims.Scoped() {
 			// The backend authoritatively knows a scoped token is the iOS
@@ -284,14 +284,14 @@ func DeriveHS256Key(secret string) []byte {
 	return sum[:]
 }
 
-// sanitizeDeviceName enforces the global ASCII-only device-name policy. Device
+// SanitizeDeviceName enforces the global ASCII-only device-name policy. Device
 // names ride in the X-Device-Name HTTP header, which can't carry non-ASCII
 // reliably (and browser fetch rejects such header values outright), so the name
 // is restricted to printable ASCII everywhere. Here we keep only printable ASCII
 // (0x20–0x7E), trim, and cap the length as a server-side backstop; clients also
 // validate the name up front for a clear error. Empty after sanitising → the
 // caller falls back to a UA-derived default.
-func sanitizeDeviceName(raw string) string {
+func SanitizeDeviceName(raw string) string {
 	var b strings.Builder
 	for _, r := range raw {
 		if r >= 0x20 && r <= 0x7E {