feat: node info, build info and acrylic web theme

Squash of recent dev work: return_session login, backend switcher, line
detail timeline, overview strips, node platform/country/ISP info with flags
and icons, build info (version, tag, time) behind site.show_build, and the
acrylic display theme.

web/display/src/BuildFooter.tsx

@@ -0,0 +1,29 @@
+import { api } from '@shared/api/client'
+import type { AboutDto, BuildDto } from '@shared/api/types'
+import { fmtBuiltAt, frontendBuild } from '@shared/buildInfo'
+import { useQuery } from '@shared/hooks'
+
+/** 「0.1.1 · v0.1.1-5-gabc123 · 2026-10-06 20:30」：版本、tag（和版本一样就不重复写）、构建时间。 */
+function buildText(b: { version: string; tag?: string | null; built_at?: string | null }): string {
+  const parts = [b.version]
+  if (b.tag && b.tag.replace(/^v/, '') !== b.version) parts.push(b.tag)
+  const at = fmtBuiltAt(b.built_at)
+  if (at) parts.push(at)
+  return parts.join(' · ')
+}
+
+/**
+ * 页面底部的构建信息：后端和前端的版本、tag、构建时间。要管理员在设置里打开「site.show_build」才显示；
+ * 关着的时候（或者后端太旧没有这个接口）什么也不画，前端自己的信息也不单独露出来。
+ */
+export function BuildFooter() {
+  const about = useQuery(() => api.get<AboutDto>('/api/public/about'), [], 5 * 60_000)
+  if (!about.data?.show_build || !about.data.backend) return null
+  const backend: BuildDto = about.data.backend
+  return (
+    <div className="build-info">
+      <span>后端 {buildText(backend)}</span>
+      <span>前端 {buildText({ version: frontendBuild.version, tag: frontendBuild.tag, built_at: frontendBuild.builtAt })}</span>
+    </div>
+  )
+}