fix(app): recover blank transcript from empty virtual window

[jpc-owner]

Problem

The session message timeline (MessageTimeline, TanStack Virtual) can render completely blank, indefinitely, on:

• cold direct navigation into a session (deep link)
• tab re-show / re-layout after the pane's flex container briefly measures 0px

Reproduces most on long sessions, which is why it's easy to miss on short ones.

Root cause

The virtualizer's scroll element can bind while it still measures clientHeight === 0
(parent flex layout not yet resolved). At that point the virtualizer caches a 0px
viewport rect and seeds its scroll offset from initialOffset
(Number.MAX_SAFE_INTEGER when anchoring to bottom). Neither self-heals:

• the cached rect only updates via the virtualizer's own ResizeObserver rect callback,
  which fires once, before rows exist, and never re-runs once the transcript populates.
• the offset only updates on a real scroll event — but scrollToEnd() /
  scrollToOffset() write element.scrollTop, which clamps to 0 on unscrollable
  content and emits no scroll event.

So getVirtualItems() stays empty forever despite getTotalSize() being non-zero:
a blank transcript that only "fixes" once the user manually scrolls (which is the
only other path that updates scrollOffset/scrollRect), and often leaves the view
mid-scroll rather than pinned to bottom afterward.

Fix

Watch the scroll element's own size with a ResizeObserver (not the virtualizer's
internal one) and, whenever it has a real height but zero virtual items are rendered,
refresh the virtualizer's cached rect from the live element and reset the scroll offset
directly (bottom-anchored or not), then remeasure. Purely additive — does not change
initialOffset / existing anchoring behavior otherwise.

Testing

• bun test in packages/app (timeline/message-timeline is not directly unit tested;
  covered by manual repro below).
• Manual repro: cold direct-link into a long session with the pane's parent briefly at
  0 height (e.g. via Forge UI embedding) — transcript now paints immediately instead of
  requiring a scroll gesture.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

The following comment was made by an LLM, it may be inaccurate: