feat(hide): 密码可见隐藏块，发帖双栏预览与门禁体验修复

增加密码解锁与游客签名 cookie；发帖页对齐 1440 并默认双栏预览；修复 locked 解析、按钮对比度与回复聚焦。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/handler/handlers.go

@@ -30,6 +30,7 @@ type Handlers struct {
 	Chat         *service.ChatService
 	Telemetry    *service.TelemetryService
 	Analytics    *service.AnalyticsService
+	HidePwd      *service.HidePasswordCookie
 }
 
 // resolvePublishStatus 决定新帖/新评的初始状态：