fix: unassigned filter not working + null guards on modal selects

- DashboardController: handle assigned_to='unassigned' before validateUserId()
  which discarded the string, causing the filter to never reach TicketModel;
  model already correctly converts 'unassigned' to IS NULL in SQL
- dashboard.js: add null guards before .value access on dynamically-created
  modal selects (bulkPriority, bulkStatus, quickStatusSelect)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/js/dashboard.js

@@ -686,7 +686,9 @@ function closeBulkPriorityModal() {
 }
 
 function performBulkPriority() {
-    const priority = document.getElementById('bulkPriority').value;
+    const priorityEl = document.getElementById('bulkPriority');
+    if (!priorityEl) return;
+    const priority = priorityEl.value;
     const ticketIds = getSelectedTicketIds();
 
     if (!priority) {
@@ -789,7 +791,9 @@ function closeBulkStatusModal() {
 }
 
 function performBulkStatusChange() {
-    const status = document.getElementById('bulkStatus').value;
+    const bulkStatusEl = document.getElementById('bulkStatus');
+    if (!bulkStatusEl) return;
+    const status = bulkStatusEl.value;
     const ticketIds = getSelectedTicketIds();
 
     if (!status) {
@@ -986,7 +990,9 @@ function closeQuickStatusModal() {
 }
 
 function performQuickStatusChange(ticketId) {
-    const newStatus = document.getElementById('quickStatusSelect').value;
+    const quickStatusEl = document.getElementById('quickStatusSelect');
+    if (!quickStatusEl) return;
+    const newStatus = quickStatusEl.value;
 
     lt.api.post('/api/update_ticket.php', { ticket_id: ticketId, status: newStatus })
     .then(data => {