The previous key was lost when .sparkle-keys directory was cleaned.
Regenerated keys using Sparkle's generate_keys tool (stores in Keychain).

New public key: BDhsLBTgtRax5K78RrmvkB2wCcLeKM7FxsuHu47soaU=

This will fix the 'improperly signed' error by ensuring the public key
in Info.plist matches the private key used to sign updates.