native: render post plaintext preview from intermediate PostContent

[davidisaaclee]

fixes TLON-4168

🧞 Hidden in this PR is a design proposition... This PR promotes the app model PostContent (different from api.PostContent) as the canonical intermediate representation for post content. https://github.com/tloncorp/tlon-apps/blob/dd0972daaca2272341cc3696030f88f03543aa72/packages/shared/src/logic/postContent.ts#L143 If accepted, we'll use this PostContent whenever interpreting a post's content; then, we'll move the API post schema to match PostContent.

What changed?

The big change here is in how the notifications render text: Black icon notification is from this PR's changes, white icon is current Testflight without this PR. iPhone/desktop in-app renderings are included for reference, since some formatting is not consistent (iPhone screenshots include this PR's change, but the screenshotted surfaces should not have been affected).

	iPhone	Desktop

The plaintext renderer is also used in the "latest post" summary:

Before this change	After this change

The renderer is a little bit configurable (in case we want to e.g. use a double space for newlines): https://github.com/tloncorp/tlon-apps/blob/6b38d5abc05ce72d33054d7e2c3afdf24608bfee/packages/shared/src/logic/postContent.ts#L145-L164

Why not update the existing getTextContent?

ub.getTextContent operated directly off of the API's post.content – part of this PR is moving to using the in-app PostContent as the canonical intermediate representation (which means we get to share logic like automatically turning list items into block-level items, and eventually having a conversion layer for different post content schemas).

How did I test?

I added a toggle to the ChatMessage.All fixture to preview how posts are rendered into plaintext:

https://github.com/user-attachments/assets/6f02c259-46b9-4ea6-9d3c-cd87233ca669

other stuff for reviewers:

• [wip] Move post content utils to shared/ is marked wip because the project will not build without the subsequent commit, but combining the commits seemed like it'd be noisy
• Delete unused code was intended to only remove the dead getTextContent and its helpers, but turned out to be a more aggressive cleanup – I did this by deleting all unused functions, including those which were exported and not imported somewhere else, which ended up removing a handful of other unused members.

[linear[bot]]

TLON-4168 Remove double spaces from plain text post content renderer

[jamesacklin]

End results look great, will leave the code review to others. Nice work!

[davidisaaclee]

Merge 1ca53e5 was a little hairy – I tested by sending **bold** *italic* as a post, and then checked the chat list "latest post" preview to ensure there was no double spacing.