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/templates/adults.tmpl

@@ -12,7 +12,7 @@
   <a href="{{.Data.PaymentsURL}}" target="_blank" rel="noopener">Payments Ledger</a>
 </div>
 
-<div class="filter-container" id="filterContainer" data-current-month="{{.Data.CurrentMonth}}" data-page="adults" data-bank-account="{{.Data.BankAccount}}">
+<div class="filter-container" id="filterContainer" data-current-month="{{.Data.CurrentMonth}}" data-page="adults" data-bank-account="{{.Data.BankAccount}}" data-months-to-show="{{.Data.MonthsToShow}}">
   <div class="filter-item">
     <label class="filter-label" for="nameFilter">Member</label>
     <input id="nameFilter" class="filter-input" type="text" placeholder="Filter by name…">

go/internal/web/templates/juniors.tmpl

@@ -12,7 +12,7 @@
   <a href="{{.Data.PaymentsURL}}" target="_blank" rel="noopener">Payments Ledger</a>
 </div>
 
-<div class="filter-container" id="filterContainer" data-current-month="{{.Data.CurrentMonth}}" data-page="juniors" data-bank-account="{{.Data.BankAccount}}">
+<div class="filter-container" id="filterContainer" data-current-month="{{.Data.CurrentMonth}}" data-page="juniors" data-bank-account="{{.Data.BankAccount}}" data-months-to-show="{{.Data.MonthsToShow}}">
   <div class="filter-item">
     <label class="filter-label" for="nameFilter">Member</label>
     <input id="nameFilter" class="filter-input" type="text" placeholder="Filter by name…">