feat: PeopleLib 开放文献客户端，集成 Z-Library 与 LibGen 等多源检索

Electron 桌面客户端，聚合多个开放获取文献源的搜索、详情与下载。

新增数据源：
- Z-Library：邮箱登录（凭据本地存储），会话失效自动重登
- LibGen：适配新版 libgen.ac 前端（旧版 search.php 镜像已全部下线）
- Memory of the World、Sci-Hub、Anna's Archive

基础设施：
- mirror.js：镜像故障转移，支持串行优先与并发竞速两种策略，
  失效镜像 5 分钟冷却后自动重试，避免站点恢复后被永久跳过
- http.js：统一 15 秒请求超时，防止单个卡死镜像拖垮整次搜索
- settings.js：全局代理配置持久化，经 Electron net.fetch 生效于所有请求

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

src/ui/util.js

@@ -0,0 +1,61 @@
+window.$ = (id) => document.getElementById(id);
+
+window.escapeHtml = (s) => String(s == null ? '' : s).replace(/[&<>"']/g, (c) => ({
+  '&': '&amp;', '<': '&lt;', '>': '&gt;', '"': '&quot;', "'": '&#39;'
+}[c]));
+
+window.coverStyle = (cover) => {
+  if (!cover) return '';
+  const url = /^(https?:|data:)/.test(cover) ? cover : 'file:///' + String(cover).replace(/\\/g, '/');
+  return `background-image:url('${url.replace(/'/g, "\\'")}')`;
+};
+
+window.copyText = async (btn, text) => {
+  await window.api.copy(text);
+  const orig = btn.textContent;
+  btn.textContent = '已复制 ✓';
+  btn.classList.add('copied');
+  setTimeout(() => { btn.textContent = orig; btn.classList.remove('copied'); }, 1500);
+};
+
+window.formatDate = (ts) => {
+  if (!ts) return '';
+  const d = new Date(ts);
+  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`;
+};
+
+window.getEnabledSources = () => {
+  let ids = null;
+  try {
+    const raw = localStorage.getItem('enabledSources');
+    if (raw) ids = JSON.parse(raw);
+  } catch (e) { /* ignore */ }
+  return ids;
+};
+
+window.setEnabledSources = (ids) => {
+  localStorage.setItem('enabledSources', JSON.stringify(ids));
+};
+
+// 通用弹窗: 返回 Promise<{ok, values}|null>
+window.openModal = (title, bodyHtml, onOk) => {
+  const modal = $('modal');
+  $('modalTitle').textContent = title;
+  $('modalBody').innerHTML = bodyHtml;
+  modal.classList.remove('hidden');
+  return new Promise((resolve) => {
+    const close = (result) => {
+      modal.classList.add('hidden');
+      $('modalOk').onclick = null;
+      $('modalCancel').onclick = null;
+      resolve(result);
+    };
+    $('modalCancel').onclick = () => close(null);
+    $('modalOk').onclick = async () => {
+      const r = onOk ? await onOk() : true;
+      if (r !== false) close(r);
+    };
+  });
+};
+
+window.confirmModal = (title, text) => window.openModal(title, `<p>${escapeHtml(text)}</p>`);