Nightly (bcbfaf3)

Rolling build from main. Not a release.

This is whatever is currently on main, built by CI. It has passed review, the
four-platform test matrix and the malware scan, and no human has used it. The
assets here are replaced every time a new nightly is cut, and this tag moves with
them. If you want something stable, take the latest release instead.

What is in this one, and not in 0.2.0

• The Build panel is now the session list. Flat, newest first, with a mode
  control to group by project instead. Reaching a conversation no longer means
  walking a hierarchy.
• The open session finally highlights in the sidebar.
• The README shows the app as it currently looks.

What has not been checked

Nobody has opened this build. The sidebar rework in particular is typechecked,
unit-tested and unseen. Visual baselines are deliberately not regenerated for it.

Nothing macOS-specific has ever been verified by a person on a Mac.

SHA256SUMS.txt is attached. It attests that a download matches what CI
uploaded; it says nothing about whether this build is any good.