bmersereau puts a leash on runaway chat-history loads

Right now, when the app shows a user their list of past chats, it pulls the whole history in one go. For a light user that is nothing; for someone who has run hundreds of conversations, every visit triggers an expensive, unbounded read. bmersereau's fix is to load chats in pages - a default batch of the 50 most recent, capped at a couple hundred, with the ability to step backward into older conversations on demand rather than all at once.

The change is written to be invisible to anyone who isn't affected: existing behaviour keeps working untouched, and users simply get their recent chats first. It is filed as a proposal to the base project and hasn't landed yet, and it targets a genuine load problem rather than cosmetics.