Additional feed performance improvements

[hjiangsu]

Pull Request Description

This PR applies some additional improvements to feed/image handling. Notably:

• The text preview is now handled within parsePost and is only computed once rather than on each build
• Improvements to logic when marking posts as read when scrolling
• Improvements to image dimension fetching using image-dimension-parser

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?