Additional comment/post page improvements

[hjiangsu]

Pull Request Description

This PR applies some additional improvements to the comment and post pages:

• Cleaned up CommentNode and its related tests.
• Fixed an issue where the comment bottom sheet modal did not show the general comment submenu when not logged in.
• Extracted the end-of-feed logic into a new widget, _PostPageFeedEnd, which only renders once the user scrolls to the bottom of the page.
• Improved scrolling performance on the post page by debouncing UpdateScrollPosition events and skipping markdown semantics when accessibility features are disabled.

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?