Configure Sparkle appcast for auto-updates

[darrylmorley]

Problem

Sparkle 2 is integrated but the appcast URL is not configured, so auto-updates don't work yet.

Tasks

• Generate EdDSA keypair with Sparkle's generate_keys tool
• Set SUFeedURL in Info.plist pointing to the appcast URL
• Create appcast.xml (host via GitHub Pages or raw repo file)
• Add EdDSA signing step to build-dmg.sh
• Upload v1.0 DMG as a GitHub Release asset
• Create first appcast entry with version, download URL, and EdDSA signature

Notes

• Sparkle 2 uses EdDSA (not DSA) for signature verification
• GitHub Releases is the simplest hosting for DMGs
• Private key must be stored securely (Keychain or secrets manager), not committed to the repo