When _loadOlderMessages prepends older messages, the viewport snaps
to the bottom instead of staying where the user was.
Two bugs compounding:
1. Wrong scrollable container. Code used `$("msgInner")` for scrollHeight
and scrollTop, but #msgInner has no overflow-y — it is a flex column.
The actual scrollable container is #messages (`.messages{overflow-y:auto}`).
Setting msgInner.scrollTop was silently ignored.
2. renderMessages calls scrollToBottom at the end (ui.js:2552),
which unconditionally scrolls #messages to the bottom and sets
_scrollPinned=true. Since bug #1 made the scroll-restore a no-op,
the page landed at the bottom every time.
Fix:
- Changed scroll restore target from `$("msgInner")` to `$("messages")`.
- Reset _scrollPinned = false after restoring the user position,
so scrollToBottom does not re-fire on next tick.
f7f8fc6496
