docs: rename contact page references to offline page in changelog

refactor(frontend): rename contact page to offline page and update routes/components

feat(frontend): add preset templates suite for offline contact page

frontend/lib/openflare/offline-page-templates/index.ts

@@ -0,0 +1,42 @@
+/**
+ * 内置离线页 HTML 模板目录。
+ */
+import baohausHtml from './baohaus';
+import lineStyleHtml from './line-style';
+import minimalistHtml from './minimalist';
+
+export type OfflinePageTemplate = {
+  /** 稳定 ID */
+  id: string;
+  /** 下拉展示名称 */
+  name: string;
+  /** 完整 HTML */
+  html: string;
+};
+
+export const OFFLINE_PAGE_TEMPLATES: OfflinePageTemplate[] = [
+  {
+    id: 'minimalist',
+    name: '极简白底',
+    html: minimalistHtml,
+  },
+  {
+    id: 'line-style',
+    name: '线框拓扑',
+    html: lineStyleHtml,
+  },
+  {
+    id: 'baohaus',
+    name: '包豪斯',
+    html: baohausHtml,
+  },
+];
+
+/** 默认预制模板 */
+export const DEFAULT_OFFLINE_PAGE_TEMPLATE_ID = 'minimalist';
+
+export function getOfflinePageTemplate(
+  id: string,
+): OfflinePageTemplate | undefined {
+  return OFFLINE_PAGE_TEMPLATES.find((item) => item.id === id);
+}