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/static/js/filters.js

@@ -12,7 +12,8 @@
   const container = document.getElementById('filterContainer');
   if (!container) return;
 
-  const currentMonth = container.dataset.currentMonth || '';
+  const currentMonth  = container.dataset.currentMonth || '';
+  const monthsToShow  = parseInt(container.dataset.monthsToShow || '0', 10);
 
   const nameInput   = document.getElementById('nameFilter');
   const fromSelect  = document.getElementById('fromMonth');
@@ -88,4 +89,10 @@
   // ── Initialise ────────────────────────────────────────────────────────────
 
   hideFutureMonths();
+  // Default the from-select to show only the last N months.
+  if (monthsToShow > 0 && toSelect.value !== '') {
+    const defaultFrom = Math.max(0, parseInt(toSelect.value, 10) - monthsToShow + 1);
+    fromSelect.value = String(defaultFrom);
+    applyMonthFilter();
+  }
 }());