fix(display): default from-selector to last N months; keep all months selectable

Instead of hiding older months entirely, show all months in the from/to
selectors but default the from-select to the last MONTHS_TO_SHOW months
on page load. The "All" button resets to full history as before.

Python: passes months_to_show to render_template, IIFE sets fromSelect.value.
Go: adds MonthsToShow to response structs, data-months-to-show attr in
templates, filters.js reads it and defaults fromSelect after hideFutureMonths.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

go/internal/web/api/build_juniors.go

@@ -136,6 +136,7 @@ func buildJuniorsResponse(
 		PaymentsURL:   "https://docs.google.com/spreadsheets/d/" + config.PaymentsSheetID + "/edit",
 		BankAccount:   cfg.QRAccount,
 		CurrentMonth:  currentMonth,
+		MonthsToShow:  cfg.MonthsToShow,
 	}
 }