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>

templates/adults.html

@@ -1045,6 +1045,7 @@
             });
 
             toSelect.value = maxMonthIdx;
+            fromSelect.value = Math.max(0, maxMonthIdx - {{ months_to_show }} + 1);
             applyMonthFilter();
         })();
     </script>

templates/juniors.html

@@ -1026,6 +1026,7 @@
             });
 
             toSelect.value = maxMonthIdx;
+            fromSelect.value = Math.max(0, maxMonthIdx - {{ months_to_show }} + 1);
             applyMonthFilter();
         })();
     </script>