Show the most recently known VO2 max value on days where Garmin has
not produced a new estimate (it only updates after certain activities).
Fix the sync lookback days input resetting to the server value during
polling — the form now initialises from the server once on first load
and is not overwritten by background refetches.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>