Improvements to create post page

[hjiangsu]

Pull Request Description

This PR improves the logic and layout for the create post page. Most of the state-related logic is now moved into the respective cubit, and I've implemented a new page which holds additional settings (e.g., flair selection, tag creation, thumbnail URL, etc.)

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?