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/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"`
 }