feat(theme+规范): 管理端接入 yh-theme 主题层并按 bsch 应用页规范升级页面骨架

- 主题层: scripts/gen-yh-theme.mjs 生成 assets/styles/yh-theme.css(构建期静态化 color-mix),
  5 主题档×侧栏深浅×渐变×side-plain 挂 html 档位类名; main.ts 于 icons.css 后引入并 initTheme() 消首帧闪色
- utils/theme.ts(+8 用例): localStorage(yh-theme) 持久化 + <html> 档位类名幂等切换
- ThemeSwitcher.vue(+6 用例): 顶栏配色胶囊(5 色网格+3 开关), 定稿 07-seam-pure 同款皮肤
- SchoolLayout: 顶栏 logo 徽标+配色切换器挂载; 站点身份色带退役, 子站橙由 yh-theme .band-sub::before 融合
- useTableHeight 响应化(+5 用例): ResizeObserver(body) 重算修复窗口缩放冻结, 常量 56/56 档不动
- bsch 规范骨架: 操作区 fix mb-12/次按钮 fr, Table 补 link-table table-bottom-box,
  文章关键字 200px 即输即搜(300ms 防抖去 enter-button), 附件分页 customPageCount 记忆+page-box fr,
  轮播展示位移入搜索区, 友链分组改 tabs-wrapper+Tabs card, 五表单校验失败统一"请填写红色星号内容！"
- 142 用例全绿, vite build 通过

src/utils/theme.ts

@@ -0,0 +1,84 @@
+/**
+ * 管理端主题状态（yh-theme，定稿 07-seam-pure.html，方案《管理端整体主题迁移方案》§6.5）。
+ * 档位全部挂在 <html>：data-theme × side-deep|side-light × grad-on|grad-off × side-plain，
+ * 皮肤规则在 assets/styles/yh-theme.css（生成物）。与门户皮肤包（templates/themes）正交。
+ */
+export const THEME_NAMES = ["blue", "ocean", "ink", "violet", "teal"] as const;
+export type ThemeName = (typeof THEME_NAMES)[number];
+
+/** 定稿默认档：blue + side-deep + grad-on + 侧栏跟随（D1 拍板 deep） */
+export interface ThemeState {
+    theme: ThemeName;
+    /** 侧栏深色（false=浅色档） */
+    sideDeep: boolean;
+    /** 顶栏渐变（联动侧栏竖向渐变） */
+    gradOn: boolean;
+    /** 侧栏颜色跟随顶栏品牌色（false=中性灰 side-plain） */
+    sideFollow: boolean;
+}
+
+export const THEME_STORAGE_KEY = "yh-theme";
+
+export const DEFAULT_THEME_STATE: ThemeState = {
+    theme: "blue",
+    sideDeep: true,
+    gradOn: true,
+    sideFollow: true,
+};
+
+/** 宽松解析：任一字段非法/缺失即回退默认档（脏数据不阻塞启动） */
+export function parseThemeState(raw: string | null): ThemeState {
+    if (!raw) return { ...DEFAULT_THEME_STATE };
+    try {
+        const o = JSON.parse(raw);
+        if (!o || typeof o !== "object") return { ...DEFAULT_THEME_STATE };
+        return {
+            theme: THEME_NAMES.includes(o.theme) ? o.theme : DEFAULT_THEME_STATE.theme,
+            sideDeep: typeof o.sideDeep === "boolean" ? o.sideDeep : DEFAULT_THEME_STATE.sideDeep,
+            gradOn: typeof o.gradOn === "boolean" ? o.gradOn : DEFAULT_THEME_STATE.gradOn,
+            sideFollow: typeof o.sideFollow === "boolean" ? o.sideFollow : DEFAULT_THEME_STATE.sideFollow,
+        };
+    } catch {
+        return { ...DEFAULT_THEME_STATE };
+    }
+}
+
+interface StorageLike {
+    getItem(k: string): string | null;
+    setItem(k: string, v: string): void;
+}
+
+export function loadThemeState(storage: StorageLike | undefined = globalThis.localStorage): ThemeState {
+    if (!storage) return { ...DEFAULT_THEME_STATE };
+    return parseThemeState(storage.getItem(THEME_STORAGE_KEY));
+}
+
+export function saveThemeState(
+    state: ThemeState,
+    storage: StorageLike | undefined = globalThis.localStorage
+): void {
+    try {
+        storage?.setItem(THEME_STORAGE_KEY, JSON.stringify(state));
+    } catch {
+        /* 隐私模式/配额失败静默：仅丢持久化，不丢本次会话生效 */
+    }
+}
+
+/** 把状态落到根元素档位类名（幂等：先清后挂），返回传入的 state 便于链式 */
+export function applyThemeState(
+    state: ThemeState,
+    root: HTMLElement = document.documentElement
+): ThemeState {
+    root.dataset.theme = state.theme;
+    root.classList.toggle("side-deep", state.sideDeep);
+    root.classList.toggle("side-light", !state.sideDeep);
+    root.classList.toggle("grad-on", state.gradOn);
+    root.classList.toggle("grad-off", !state.gradOn);
+    root.classList.toggle("side-plain", !state.sideFollow);
+    return state;
+}
+
+/** 启动引导：main.ts 于挂载前调用，消除首帧闪色 */
+export function initTheme(): ThemeState {
+    return applyThemeState(loadThemeState());
+}