feat: SSR 站点单页 /page/:slug 与 Admin CRUD

EOF

Co-authored-by: Cursor <cursoragent@cursor.com>

templates/admin/nav.tmpl

@@ -4,6 +4,7 @@
   <a href="/admin/boards"{{if eq .NavActive "boards"}} class="is-active"{{end}}>板块</a>
   <a href="/admin/moderation"{{if eq .NavActive "moderation"}} class="is-active"{{end}}>审核</a>
   <a href="/admin/friend-links"{{if eq .NavActive "friend-links"}} class="is-active"{{end}}>友链</a>
+  <a href="/admin/pages"{{if eq .NavActive "pages"}} class="is-active"{{end}}>单页</a>
   <a href="/admin/settings"{{if eq .NavActive "settings"}} class="is-active"{{end}}>设置</a>
   <a href="/">返回前台</a>
 </nav>