Compare gains a time-of-year filter it never had; the calendar's flat 4-season
filter is upgraded to the same model so both pages filter time identically.
- Shared season/month model in shared.js: seasons are the primary, color-coded
selector, the 12 months a nested suboption, backed by one Set of month indices.
seasonFilterDropdown/syncSeasonChecks/applySeasonMonthChange/seasonSummaryText
plus month<->bitmask helpers, reused by both pages.
- Compare: filter gates r.date's month into computeStats and the distribution;
instant re-rank, no refetch. Persisted (cmpMonths bitmask) and carried in the
URL hash (m=, omitted when all 12). Empty-selection hint when a filter excludes
every day.
- Calendar: checkedSeasons -> checkedMonths, applied in the render loop; persisted
as calMonths. Season checkbox is checked/indeterminate by how many of its months
are on.
- Mobile filter sheet (filtersheet.js): a subtle floating pill slides the page's
filter panel up as a bottom sheet, reachable from anywhere on the page, on both
compare and calendar. Desktop keeps the inline panel unchanged. Respects
reduced-motion and the home-indicator inset.
Frontend-only; no backend or payload change.
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