User Activity report: filter 'Last Activity' by the selected date range (#52)

The last_activity subquery had no WHERE clause on the report's
date-range filter, so it always showed true all-time last activity
even when the page was filtered to e.g. "last 7 days" — inconsistent
with every other column on the same report. Added the same
DATE(created_at) BETWEEN ? AND ? clause used by the report's other
subqueries.

Verified against a local MariaDB instance: an out-of-range audit_log
row is correctly excluded from last_activity once the filter is
applied.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MGDKHiU5RJdo3dqQUDow3X

index.php

@@ -391,13 +391,16 @@ switch (true) {
                 LEFT JOIN (
                     SELECT user_id, MAX(created_at) as last_activity
                     FROM audit_log
+                    WHERE DATE(created_at) BETWEEN ? AND ?
                     GROUP BY user_id
                 ) al ON u.user_id = al.user_id
                 ORDER BY tickets_created DESC, tickets_resolved DESC";
 
         $stmt = $conn->prepare($sql);
         $stmt->bind_param(
-            'ssssssss',
+            'ssssssssss',
+            $dateRange['from'],
+            $dateRange['to'],
             $dateRange['from'],
             $dateRange['to'],
             $dateRange['from'],