feat: virtualized scrolling + paginated message loading for long sessions (#8535)

[CasualDeveloper]

Following up on feedback from #6138 - @rekram1-node mentioned virtualized scrolling would be the better long-term solution.

Problem

Loading thousands of messages at once is slow and memory-heavy.

Proposed approach

1. Paginated fetch - load messages in chunks as user scrolls up
2. Virtualized rendering - only render visible messages, placeholder the rest

Questions for maintainers

• Is this something you'd want contributed, or is it already on the roadmap?
• Any pointers on how virtualization fits with opentui/SolidJS?

Happy to take a stab at this with guidance.