Commit 5800179 (multi-day calendar) accidentally removed stuck-loading
recovery. Re-add an 8-second watchdog loop in MenuBarContent that calls
recoverFromStuckLoading(). Change inFlightKeys from Set to timestamped
Dict so the watchdog can detect orphaned quiet-refresh entries. Stop
nuking payloadRefreshGeneration in the watchdog (it was killing healthy
in-flight fetches). Bump Package.swift platform to macOS 15 to match
NSAttributedString(attachment:) usage.