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/service/url_style_test.go

@@ -0,0 +1,112 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/freefire/jiang13-bbs/model"
+	"github.com/glebarez/sqlite"
+	"gorm.io/gorm"
+)
+
+func newUrlStyleTestService(t *testing.T) *SettingService {
+	t.Helper()
+	db, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{})
+	if err != nil {
+		t.Fatalf("open sqlite: %v", err)
+	}
+	if err := db.AutoMigrate(&model.SiteSetting{}); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	return NewSettingService(db)
+}
+
+func TestUrlStyleDefaultWhenMissing(t *testing.T) {
+	s := newUrlStyleTestService(t)
+	got, err := s.UrlStyle()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != DefaultUrlStyle {
+		t.Fatalf("missing row should fall back to %q, got %q", DefaultUrlStyle, got)
+	}
+	pub, err := s.Public()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if pub.UrlStyle != DefaultUrlStyle {
+		t.Fatalf("Public().UrlStyle = %q, want %q", pub.UrlStyle, DefaultUrlStyle)
+	}
+}
+
+func TestSetUrlStyleRoundTrip(t *testing.T) {
+	s := newUrlStyleTestService(t)
+	for _, style := range []string{"html", "discuz"} {
+		if err := s.SetUrlStyle(style); err != nil {
+			t.Fatalf("SetUrlStyle(%q): %v", style, err)
+		}
+		got, err := s.UrlStyle()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got != style {
+			t.Fatalf("UrlStyle() = %q, want %q", got, style)
+		}
+		pub, err := s.Public()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if pub.UrlStyle != style {
+			t.Fatalf("Public().UrlStyle = %q, want %q", pub.UrlStyle, style)
+		}
+	}
+}
+
+func TestSetUrlStyleDefaultDeletesKey(t *testing.T) {
+	s := newUrlStyleTestService(t)
+	if err := s.SetUrlStyle("html"); err != nil {
+		t.Fatal(err)
+	}
+	// 传 default 应删键，恢复「缺行=默认」语义（大小写与空白容错）
+	if err := s.SetUrlStyle("  DEFAULT "); err != nil {
+		t.Fatal(err)
+	}
+	got, err := s.UrlStyle()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != DefaultUrlStyle {
+		t.Fatalf("after reset got %q, want %q", got, DefaultUrlStyle)
+	}
+	var rows int64
+	s.db.Where("key = ?", SettingKeyUrlStyle).Model(&model.SiteSetting{}).Count(&rows)
+	if rows != 0 {
+		t.Fatalf("default should delete the row, rows=%d", rows)
+	}
+}
+
+func TestSetUrlStyleRejectsInvalid(t *testing.T) {
+	s := newUrlStyleTestService(t)
+	for _, bad := range []string{"", "pretty", "HTML5", "thread", "../etc"} {
+		if err := s.SetUrlStyle(bad); err == nil {
+			t.Fatalf("SetUrlStyle(%q) should fail", bad)
+		}
+	}
+	// 非法值不应落库
+	if _, err := s.UrlStyle(); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestUrlStyleInvalidStoredValueFallsBack(t *testing.T) {
+	s := newUrlStyleTestService(t)
+	if err := s.putValue(SettingKeyUrlStyle, "nonsense"); err != nil {
+		t.Fatal(err)
+	}
+	got, err := s.UrlStyle()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != DefaultUrlStyle {
+		t.Fatalf("invalid stored value should fall back to %q, got %q", DefaultUrlStyle, got)
+	}
+}