Add Sparkle in-app updates, bump to 2.1

- Replace the GitHub-polling UpdateChecker with Sparkle 2: EdDSA-signed
  appcast feed, in-place install, background checks every 6 hours
- New AppUpdater wrapper; Check for App Updates menu item and About button
- build-dmg.sh: sign Sparkle's nested XPC services and helpers with our
  Developer ID (hardened runtime + timestamp) so notarization passes,
  and generate appcast.xml after notarization
- MARKETING_VERSION 2.1, build 11; README and CHANGELOG updated
- Fold the unreleased 2.0.3 notes into the 2.1 changelog entry (2.0.2 is
  the latest published release, so 2.1 ships those changes first)

Release TopOff v2.0.2

Bump to 2.0.2 (build 9), add the v2.0.2 changelog entry, and point the
README download links at the new DMG.

Release TopOff v2.0.1

Bump to 2.0.1 (build 8), add the v2.0.1 changelog entry, and point the
README download links at the new DMG.

TopOff v2.0 — signed and notarized release

TopOff v1.8.3

Release TopOff v1.8.2

Release TopOff v1.8.1

TopOff v1.8

Release v1.7.1

TopOff v1.7