Only the month view is rendered anywhere, so drop the unused year
calendar, the size variants and the helpers that existed for them.
- Build the month grid in one place (monthDays.ts) with the week-start
rotation and out-of-month padding, and test it.
- Keep every chip visual in cellStyles.ts. A selected day now owns its
fill outright so the empty-cell hover tint cannot swap the accent out.
- MonthCalendar derives maxCount from its own data instead of taking it
as a prop; the index exports only what callers use.
