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/config/config_test.go

@@ -24,9 +24,22 @@ func TestValidate_DevWithDevTokenOK(t *testing.T) {
 }
 
 func TestValidate_ProdOK(t *testing.T) {
-	c := &Config{AuthMode: "prod", OIDCAudience: "cdrop-web,cdrop-desktop"}
+	c := &Config{AuthMode: "prod", OIDCAudience: "cdrop-web,cdrop-desktop", SessionSecret: "s3cr3t"}
 	if err := c.validate(); err != nil {
-		t.Fatalf("prod mode with audience should pass; got %v", err)
+		t.Fatalf("prod mode with audience + session secret should pass; got %v", err)
+	}
+}
+
+func TestValidate_ProdRequiresSessionSecret(t *testing.T) {
+	// SessionSecret keys at-rest encryption of browser refresh_tokens; without it
+	// passwordless re-login can't run safely. Refuse to boot in prod.
+	c := &Config{AuthMode: "prod", OIDCAudience: "cdrop-web", SessionSecret: ""}
+	err := c.validate()
+	if err == nil {
+		t.Fatal("prod without CDROP_SESSION_SECRET must reject; got nil error")
+	}
+	if !strings.Contains(err.Error(), "CDROP_SESSION_SECRET") {
+		t.Errorf("error must mention CDROP_SESSION_SECRET; got %q", err.Error())
 	}
 }