Make FollowMyFriends startup durable

[bryangauvin]

Summary

• Auto-start tracking after key validation when launchAtStartup is enabled
• Trigger an immediate poll when tracking starts so login launches publish promptly
• Prevent MQTT password Keychain access from blocking startup by using noninteractive access with a 0600 local fallback password file

Validation

• Built with xcodebuild
• Installed to ~/Applications/FollowMyFriends.app
• Verified LaunchAgent starts the installed app
• Verified logs show tracking started, MQTT connected, WAL watcher active, Find My read 4 friends, and MQTT published all 4 trackers
• Verified MQTT retained attributes read back for amy, aurora, bryan, and phoenix

Note: local repo has unrelated pre-existing dirty home-zone changes that are not included in this PR.