task-15(壳层/路由): 实现页面标题与面包屑规则

title-breadcrumb 提供 resolveDocumentTitle('页面 - 数富AI')、updateDocumentTitle
(非浏览器静默)、crumbsForActiveRoute(分组->页面)；AdminLayout 用 watch 同步
document.title 并在层级>1 时渲染 el-breadcrumb。

admin-frontend-vue/src/layout/title-breadcrumb.ts

@@ -0,0 +1,42 @@
+import type { AdminMenuNode } from '../types/admin'
+
+export const ADMIN_BRAND = '数富AI'
+
+/** 文档标题规则：`页面标题 - 数富AI`；无页面标题时仅品牌。 */
+export function resolveDocumentTitle(pageTitle: string, brand: string = ADMIN_BRAND): string {
+  const trimmed = (pageTitle || '').trim()
+  return trimmed ? `${trimmed} - ${brand}` : brand
+}
+
+/** 同步 HTML 文档标题；doc 为空(如非浏览器环境)时静默跳过。 */
+export function updateDocumentTitle(title: string, doc: { title: string } | null | undefined): void {
+  if (doc) doc.title = title
+}
+
+export interface BreadcrumbItem {
+  key: string
+  name: string
+  route?: string
+}
+
+/**
+ * 面包屑规则：在菜单树中定位激活路由，返回“分组 -> 页面”层级（顶层叶子只返回自身）。
+ * 无命中返回空数组，不伪造路径。
+ */
+export function crumbsForActiveRoute(nodes: AdminMenuNode[] | null | undefined, activeRoute: string): BreadcrumbItem[] {
+  for (const node of nodes || []) {
+    if (node.route === activeRoute) return [{ key: node.key, name: node.name, route: node.route }]
+    const children = node.children || []
+    for (const child of children) {
+      if (child.route === activeRoute) {
+        return [
+          { key: node.key, name: node.name },
+          { key: child.key, name: child.name, route: child.route },
+        ]
+      }
+    }
+    const nested = crumbsForActiveRoute(children, activeRoute)
+    if (nested.length) return [{ key: node.key, name: node.name }, ...nested]
+  }
+  return []
+}