余白 UI：按设计稿模块化还原 + 站主自定义 CSS + 详情 neighbors + 静态缓存头

前端（新增 ui/yohaku/，各视图按 ui_id=vivid 分支渲染；classic 与后台不受影响）
- 令牌层：设计稿数值原样落地，28 个 --v-* 为站主接口（自定义 CSS 可覆盖，
  放 @layer 让出优先级）；另加防线，中和 styles.css 裸选择器
  （h1 衬线 / body 行高 / 后台 .stat/.toolbar/.chip 卡片）漏进余白元素
- 顶栏（滚动收缩态）/ 阅读进度（顶部 2px + READ n%）/ 页脚
- 首页：hero（统计条）/ 筛选（类型分段 + 标签 chips + 搜索）/
  长文卡片与短文纸条 / 分页 / 作品展示区（3 件 + 查看所有，参照 diygod.cc）；
  面板栅格改 1.6fr 1fr 1fr 且高度自适应内容（标签云明显大一圈）
- 文章页：三栏「纸」—— 左时间线（前后各两篇）/ 中纸张 / 右目录与阅读进度；
  短文详情不显示标题（h1 视觉隐藏保留无障碍）
- 全部规则收敛在 html[data-ui='vivid'] 前缀下；无 !important

后端
- GET /api/posts/:slug 增 neighbors（前后各两篇，详情页侧栏时间线用）
- SPA 缓存头：index.html no-cache，assets/ 带 hash 长期 immutable
  （此前无缓存头，浏览器会一直拿旧构建）

其他
- 站主自定义 CSS 管线（customCss.js + ui/sections.js，按分区注入，仅 vivid）
- 删除旧 vivid 覆盖层实现（VividNav/Hero/Footer/Timeline/Toc、ui/vivid.css）
- 设计稿原型附于仓库根 index.html；作品表清掉外链 diygod.cc 的示例封面，
  改用本地占位图（public/covers/）

backend/internal/store/store_test.go

@@ -113,6 +113,122 @@ func TestSettingsLightSkinFallback(t *testing.T) {
 	}
 }
 
+func TestSettingsUIFallback(t *testing.T) {
+	s := openTestStore(t)
+
+	st, err := s.GetSettings()
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if st.UIID != UIClassic {
+		t.Errorf("default ui_id should be %q, got %q", UIClassic, st.UIID)
+	}
+	if st.CustomCSS == nil {
+		t.Error("CustomCSS should never be nil")
+	}
+
+	// An unknown ui_id is coerced to the default on write.
+	st.UIID = "bogus"
+	if err := s.UpdateSettings(st); err != nil {
+		t.Fatalf("update: %v", err)
+	}
+	got, _ := s.GetSettings()
+	if got.UIID != UIClassic {
+		t.Errorf("bogus ui_id should fall back to %q, got %q", UIClassic, got.UIID)
+	}
+
+	// A valid one persists.
+	st.UIID = UIVivid
+	if err := s.UpdateSettings(st); err != nil {
+		t.Fatalf("update again: %v", err)
+	}
+	got, _ = s.GetSettings()
+	if got.UIID != UIVivid {
+		t.Errorf("ui_id should persist, got %q", got.UIID)
+	}
+}
+
+func TestSettingsUIFallbackOnRead(t *testing.T) {
+	s := openTestStore(t)
+	// A hand-edited / legacy row must not leak an unknown UI to clients.
+	if _, err := s.db.Exec(s.db.Q(`INSERT INTO settings(key,value) VALUES (?,?) ON CONFLICT(key) DO UPDATE SET value=excluded.value`),
+		"ui_id", "neon"); err != nil {
+		t.Fatalf("seed ui_id: %v", err)
+	}
+	st, err := s.GetSettings()
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if st.UIID != UIClassic {
+		t.Errorf("expected read-side fallback to %q, got %q", UIClassic, st.UIID)
+	}
+}
+
+func TestSettingsCustomCSSRoundTrip(t *testing.T) {
+	s := openTestStore(t)
+
+	// Seeding a raw row with an unknown section + a valid one.
+	if _, err := s.db.Exec(s.db.Q(`INSERT INTO settings(key,value) VALUES (?,?) ON CONFLICT(key) DO UPDATE SET value=excluded.value`),
+		"custom_css", `{"home":"body{--v-accent:#f0f}","bogus":"x{}"}`); err != nil {
+		t.Fatalf("seed custom_css: %v", err)
+	}
+	st, _ := s.GetSettings()
+	if st.CustomCSS["home"] != "body{--v-accent:#f0f}" {
+		t.Errorf("home css not read back: %+v", st.CustomCSS)
+	}
+	if _, ok := st.CustomCSS["bogus"]; ok {
+		t.Error("unknown section should be dropped on read")
+	}
+
+	// Write side: unknown sections dropped, XML-hostile chars survive, nil clears.
+	st.CustomCSS = map[string]string{
+		"home":  `a::after{content:"<&>"}`,
+		"bogus": "x{}",
+	}
+	if err := s.UpdateSettings(st); err != nil {
+		t.Fatalf("update: %v", err)
+	}
+	got, _ := s.GetSettings()
+	if got.CustomCSS["home"] != `a::after{content:"<&>"}` {
+		t.Errorf("css mangled in round trip: %q", got.CustomCSS["home"])
+	}
+	if len(got.CustomCSS) != 1 {
+		t.Errorf("unknown section should be dropped on write, got %+v", got.CustomCSS)
+	}
+
+	// A nil map is a full clear, and still comes back as a non-nil empty map.
+	got.CustomCSS = nil
+	if err := s.UpdateSettings(got); err != nil {
+		t.Fatalf("clear: %v", err)
+	}
+	after, _ := s.GetSettings()
+	if after.CustomCSS == nil {
+		t.Error("CustomCSS should be non-nil after clearing")
+	}
+	if len(after.CustomCSS) != 0 {
+		t.Errorf("expected cleared map, got %+v", after.CustomCSS)
+	}
+}
+
+func TestSettingsCustomCSSOverLongSectionDropped(t *testing.T) {
+	s := openTestStore(t)
+	st, _ := s.GetSettings()
+	st.CustomCSS = map[string]string{
+		"home":  strings.Repeat("a", maxSectionCSSBytes+1),
+		"about": "b{color:red}",
+	}
+	if err := s.UpdateSettings(st); err != nil {
+		t.Fatalf("update: %v", err)
+	}
+	got, _ := s.GetSettings()
+	if _, ok := got.CustomCSS["home"]; ok {
+		t.Error("over-long section should be dropped")
+	}
+	if got.CustomCSS["about"] != "b{color:red}" {
+		t.Errorf("valid section should survive, got %+v", got.CustomCSS)
+	}
+}
+
 // TestStoreSQLite runs a small integration pass on top of an in-memory SQLite
 // database to exercise the new fields (cover_url, tag color) and the merge
 // + bulk + dashboard paths.