fix(app): paint cold deep-link timeline after viewport layout

[maschwenk]

Summary

• Fixes blank conversation on cold direct navigation (e.g. Forge task deep links) when the messages API returns quickly.
• Root cause: TanStack Virtual attached scroll observers while the scroll viewport still had height 0, so outerSize stayed 0 and getVirtualItems() stayed empty despite a non-zero totalSize.
• Bind ScrollView viewportRef synchronously; drop Number.MAX_SAFE_INTEGER initialOffset; remeasure/re-anchor once the scroll root has a real height (including forcing scrollRect when observers miss the layout settle).

Test plan

• Hard-navigate to a Forge task URL with a ready OpenCode session and confirm the transcript paints without throttling
• Click away to another task and back; transcript still paints
• Slow the messages API (~5s) and confirm transcript still paints (previous workaround path)

Made with Cursor

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