Fix chat history pagination and scroll preservation
Align session history loading with the expected scroll-up pagination UX while
keeping OpenChamber-specific initial message limits for constrained runtimes.
- Separate initial load sizes from older-history pagination size
- Load older messages automatically when scrolling near the top
- Continue fetching history until a visible older turn is available
- Preserve the current viewport synchronously during prepends
- Prevent history loading from fighting pinned-to-bottom follow behavior
- Remove delayed scroll-to-bottom correction that caused jumpbacks
- Fix the virtualizer fallback path that could render a large blank spacer
- Track oldest loaded message per pagination iteration to avoid redundant fetches
Bohdan Triapitsyn
f9e9f30873