Support infinite scroll for message history

[m96-chan]

Description

Message history is currently limited to a configurable number of messages per channel (default 50, max 100). There is no way to load older messages beyond this limit.

Proposed Solution

Implement backward pagination for message history:

• When the user scrolls to the top of the message list, automatically fetch the next batch of older messages via conversations.history with a cursor
• Append older messages above the current list
• Show a loading indicator while fetching
• Consider capping total in-memory messages to avoid excessive memory usage

Impact

Essential for browsing conversation history. Users frequently need to scroll back through long discussions.