[WIP] Add fallback behavior for back button on PageHeader

[Codex]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>页面栈为1时无法返回时自动跳转首页/详情页</issue_title>
<issue_description>当页面栈仅剩1时，（PageHeader的）返回按钮无效。这种情况下应该自动跳转首页；如果是 Anchor PR Page 则跳转到所属活动页（理论上应该是替换当前栈，避免历史记录不符合直觉）。具体实现时，应该是扩展 PageHeader 的行为，并且允许自定义该行为 (prop:backFallbackTo)</issue_description>

Comments on the Issue (you are @codex[agent] in this section)

• Fixes 页面栈为1时无法返回时自动跳转首页/详情页 #44