fix: restore Sep+Oct adult merge and stop auto-truncating period selector

Two regressions made older periods invisible on the adults dashboard:

- 1257f0d (Mar 9) commented out ADULT_MERGED_MONTHS, removing the
  Sep+Oct 2025 merged label. Restored only the 2025-09 → 2025-10
  mapping (Dec and Jan are billed separately for adults; the
  Dec → Jan mapping stays disabled per product decision). Mirrored
  on the Go side. Test fixtures in sources_test.go now assert Sep
  dates land in merged 2025-10 instead of 2025-09.

- 7774301 (Apr 9) added a JS onload default that set the From
  selector to maxMonthIdx − 4 and immediately filtered the table,
  hiding everything older than 5 months on first load. Dropped that
  default in templates/adults.html and templates/juniors.html so
  the From-selector starts at the oldest available month. Future
  months are still removed from the dropdowns and hidden in the
  table — only the past-month truncation is gone.

Note: the live adults attendance sheet had also been pruned to
start at 02.12.2025; restoring Sep/Oct/Nov 2025 columns from
Sheets version history is required to actually see those periods.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/services/membership/sources_test.go

@@ -46,8 +46,8 @@ func TestLoadAdults(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	// adultMergedMonths is empty so 2025-09 stays as-is
-	if len(months) != 1 || months[0] != "2025-09" {
+	// AdultMergedMonths sends 2025-09 → 2025-10
+	if len(months) != 1 || months[0] != "2025-10" {
 		t.Errorf("unexpected months: %v", months)
 	}
 	if len(members) != 2 {
@@ -55,7 +55,7 @@ func TestLoadAdults(t *testing.T) {
 	}
 	byName := map[string]int{}
 	for _, m := range members {
-		byName[m.Name] = m.Fees["2025-09"].Attendance
+		byName[m.Name] = m.Fees["2025-10"].Attendance
 	}
 	if byName["Alice"] != 2 {
 		t.Errorf("Alice: want 2 sessions, got %d", byName["Alice"])
@@ -73,9 +73,9 @@ func TestLoadAdults_Fee(t *testing.T) {
 	}
 	byName := map[string]int{}
 	for _, m := range members {
-		byName[m.Name] = m.Fees["2025-09"].Expected
+		byName[m.Name] = m.Fees["2025-10"].Expected
 	}
-	// 2 sessions in 2025-09 → AdultFeeMonthlyRate["2025-09"] = 750
+	// 2 sessions land in merged 2025-10 → AdultFeeMonthlyRate["2025-10"] = 750
 	if byName["Alice"] != 750 {
 		t.Errorf("Alice fee: want 750, got %d", byName["Alice"])
 	}