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/adults.go

@@ -39,4 +39,5 @@ type AdultsResponse struct {
 	PaymentsURL   string                      `json:"payments_url"`
 	BankAccount   string                      `json:"bank_account"`
 	CurrentMonth  string                      `json:"current_month"`
+	MonthsToShow  int                         `json:"months_to_show"`
 }

go/internal/web/api/build_adults.go

@@ -140,6 +140,7 @@ func buildAdultsResponse(
 		PaymentsURL:   "https://docs.google.com/spreadsheets/d/" + config.PaymentsSheetID + "/edit",
 		BankAccount:   cfg.QRAccount,
 		CurrentMonth:  currentMonth,
+		MonthsToShow:  cfg.MonthsToShow,
 	}
 }
 

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,
 	}
 }
 

go/internal/web/api/handler.go

@@ -53,7 +53,7 @@ func (h *Handler) AssembleAdults(ctx context.Context) (AdultsResponse, error) {
 		return AdultsResponse{}, err
 	}
 	result := domreconcile.Reconcile(members, sortedMonths, txns, exceptions, time.Now().Year())
-	return buildAdultsResponse(members, lastNMonths(sortedMonths, h.Config.MonthsToShow), result, txns, h.Config, time.Now().Format("2006-01")), nil
+	return buildAdultsResponse(members, sortedMonths, result, txns, h.Config, time.Now().Format("2006-01")), nil
 }
 
 // ServeJuniors handles GET /api/juniors.
@@ -74,16 +74,7 @@ func (h *Handler) AssembleJuniors(ctx context.Context) (JuniorsResponse, error)
 		return JuniorsResponse{}, err
 	}
 	result := domreconcile.Reconcile(members, sortedMonths, txns, exceptions, time.Now().Year())
-	return buildJuniorsResponse(members, lastNMonths(sortedMonths, h.Config.MonthsToShow), result, txns, h.Config, time.Now().Format("2006-01")), nil
-}
-
-// lastNMonths returns the last n elements of months.
-// If n <= 0 or n >= len(months), the full slice is returned unchanged.
-func lastNMonths(months []string, n int) []string {
-	if n > 0 && len(months) > n {
-		return months[len(months)-n:]
-	}
-	return months
+	return buildJuniorsResponse(members, sortedMonths, result, txns, h.Config, time.Now().Format("2006-01")), nil
 }
 
 // ServePayments handles GET /api/payments.

go/internal/web/api/juniors.go

@@ -38,4 +38,5 @@ type JuniorsResponse struct {
 	PaymentsURL   string                       `json:"payments_url"`
 	BankAccount   string                       `json:"bank_account"`
 	CurrentMonth  string                       `json:"current_month"`
+	MonthsToShow  int                          `json:"months_to_show"`
 }