Improve inserting a new item at the end of a list via Alt+Enter

[TCOTC]

• 按 Alt+Enter 之后，如果当前列表项块中没有子列表块，则在最后一个子块的后面新建列表块 https://ld246.com/article/1762643983269
• 把通过 .lastElementChild.previousElementSibling 获取最后一个子块的方式修改为通过新增的 getLastChildBlock 函数获取，也许能解决使用一些主题或插件时编辑列表会导致状态异常的情况（主题或插件会在编辑器中插入自定义的元素，导致 .lastElementChild.previousElementSibling 获取到的不是最后一个子块）

[Copilot]

Pull request overview

This PR updates the Protyle editor’s list and hotkey behavior to better support inserting list sub-items (Alt+Enter) and to make “last child block” detection more robust by centralizing block-element checks.

Changes:

• Added isBlockElement() helper and refactored callers to use it.
• Introduced getLastChildBlock() and replaced multiple lastElementChild.previousElementSibling usages in list logic.
• Reworked Alt+Enter handling to prioritize code-block language selection, list sub-item insertion, and callout type/title updates.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
app/src/protyle/wysiwyg/list.ts	Adds getLastChildBlock() and reworks sub-list insertion / indent-outdent logic to use it.
app/src/protyle/wysiwyg/keydown.ts	Refactors Alt+Enter dispatch between code blocks, lists, and callouts.
app/src/protyle/util/hasClosest.ts	Adds isBlockElement() and uses it inside hasClosestBlock().

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[Copilot]

previousLastBlock can be null when getLastChildBlock(previousElement) fails to find any Node* child (e.g. a list item temporarily/accidentally containing only .protyle-action + .protyle-attr). In that case this branch will throw on previousLastBlock.getAttribute(...). Add a null-guard here (and decide on a fallback insertion point / foldElement behavior) before accessing attributes on previousLastBlock.