feat: implement Sparkle auto-update support

[parkergenix]

Summary

• Wires up the already-linked Sparkle framework so the app can check for and install updates
• Adds SPUStandardUpdaterController to AppDelegate, started at launch
• Adds a Check for Updates... item to the right-click context menu
• Adds SUFeedURL and SUPublicEDKey entries to Info.plist

What the maintainer needs to do before merging

1. Generate your EdDSA key pair by running generate_keys from Sparkle's bin/ folder (found in DerivedData after building once):

   ./SourcePackages/artifacts/sparkle/Sparkle/bin/generate_keys
   

2. Replace the SUPublicEDKey placeholder in Info.plist with the output public key
3. Keep the private key in your Keychain — use sign_update from the same bin/ to sign each release .zip
4. Host an appcast.xml at the SUFeedURL (defaulted to https://raw.githubusercontent.com/zephkelly/rtaudio/main/appcast.xml — update if your path differs)

Sparkle's documentation covers the full release signing workflow.

[ZephyrCodesStuff]

Need to merge as GitHub Actions won't inject the repo's secret Sparkle key on runs from forked PRs. I'll try to take it from here; thank you for the initial implementation!