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/shared/buildInfo.ts

@@ -0,0 +1,21 @@
+/** 前端自己的构建信息（构建时由 Vite 写进来，见 web/build-info.mjs）。 */
+declare const __APP_VERSION__: string
+declare const __APP_TAG__: string
+declare const __APP_COMMIT__: string
+declare const __APP_BUILT_AT__: string
+
+export const frontendBuild = {
+  version: __APP_VERSION__,
+  tag: __APP_TAG__,
+  commit: __APP_COMMIT__,
+  builtAt: __APP_BUILT_AT__,
+}
+
+/** 构建时间显示成本地时间「2026-10-06 20:30」；没有（开发服务器）就是空。 */
+export function fmtBuiltAt(iso: string | null | undefined): string {
+  if (!iso) return ''
+  const d = new Date(iso)
+  if (Number.isNaN(d.getTime())) return iso
+  const p = (n: number) => String(n).padStart(2, '0')
+  return `${d.getFullYear()}-${p(d.getMonth() + 1)}-${p(d.getDate())} ${p(d.getHours())}:${p(d.getMinutes())}`
+}