feat: 可配置站点 URL 风格

- 新增 urlStyle 工具与测试：postPath/userPath/sitePagePath 统一生成链接
- 管理端新增 UrlStylePanel 设置面板，settings 服务与 handler 适配
- 全站路由链接与 JSON-LD/sitemap 切换为统一 URL 生成
- middleware 与相关组件适配

backend/handler/operations.go

@@ -245,8 +245,12 @@ func (h *Handlers) SiteState(c *gin.Context) {
 		c.JSON(503, gin.H{"error": "状态暂不可用"})
 		return
 	}
+	urlStyle, e := h.Setting.UrlStyle()
+	if e != nil {
+		urlStyle = service.DefaultUrlStyle
+	}
 	c.Header("Cache-Control", "no-store")
-	c.JSON(200, gin.H{"allow_register": cfg.AllowRegister, "register_notice": cfg.RegisterNotice, "verify_email": cfg.VerifyEmail, "password_reset": cfg.PasswordReset, "maintenance": m, "bypass": h.administrator(c), "site_url": h.Cfg.SiteURL})
+	c.JSON(200, gin.H{"allow_register": cfg.AllowRegister, "register_notice": cfg.RegisterNotice, "verify_email": cfg.VerifyEmail, "password_reset": cfg.PasswordReset, "maintenance": m, "bypass": h.administrator(c), "site_url": h.Cfg.SiteURL, "url_style": urlStyle})
 }
 func (h *Handlers) SendEmailCode(c *gin.Context) {
 	var req struct {