Fix totals footer painting behind sticky group-row headers on scroll

tfoot had position: sticky but no explicit z-index (auto/0), while the
sticky month/year group-row headers use z-index: 1 -- so a group row
scrolling past the bottom painted on top of the totals footer instead
of behind it. z-index: 3 (above thead's 2 and the group-row's 1).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/modules/custom/figli_compta_ledger/css/home.css

@@ -132,6 +132,10 @@ html.gin--dark-mode #figli-home-app {
 #figli-home-app tfoot {
   position: sticky;
   bottom: 0;
+  /* Higher than thead (2) and the sticky group-row headers (1) -- without
+     this, tfoot had no explicit z-index (auto), so a group-row header
+     scrolling underneath it painted on top instead of behind. */
+  z-index: 3;
 }
 
 #figli-home-app tr.figli-totals-row td {