Unify font-size and body width/margins between PostPreview and PostView

This should ensure that if the user starts reading the excerpt, after opening the post, they can follow where they left off without having to find the continuation.

We could also enhance this by using [View Transitions](https://developer.mozilla.org/en-US/docs/Web/API/View_Transition_API)

Start by locating the PostPreview and PostView components and compare their font-size, body width, and margin styles. Confirm the excerpt and opened post use matching layout values so reading can continue seamlessly; treat View Transitions as a separate enhancement if its scope is clarified.

Written by the indexing model from the issue text.