v0.6.0

v0.6.0: in-app self-update from the About panel

About → 'Update now' downloads the latest release, verifies it, swaps the app in
place, and relaunches — no manual drag-to-Applications, no admin (the app is
user-owned).

- src/selfupdate.py: download (certifi HTTPS) → mount → VERIFY (spctl notarized
  + codesign TeamIdentifier pinned to ZWXAL8XA46; a tampered/fake update can't
  install) → stage → detached helper waits for quit, ditto-swaps the bundle,
  clears quarantine, relaunches. Falls back to the release page when running
  from source or a read-only install.
- src/updates.py: expose the DMG asset browser_download_url.
- src/app.py: About shows 'Update now to vX' when a self-update can apply, else
  the old 'Download update' page button.
- tests/test_selfupdate.py; README documents it; version → 0.6.0.

Verified end-to-end: a running v0.6.0 downloaded the notarized v0.5.0, verified
it, replaced itself and relaunched; verify correctly rejects wrong-team and
non-notarized apps.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>