Use styled 404 page for missing/inaccessible tickets

Replaces the bare "Ticket not found" text response with the shared
views/error_404.php partial so users see the full TDS-styled error page.
Also collapsed the two identical 404 branches into one check.

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

controllers/TicketController.php

@@ -36,16 +36,9 @@ class TicketController {
         // Get ticket data
         $ticket = $this->ticketModel->getTicketById($id);
 
-        if (!$ticket) {
-            header("HTTP/1.0 404 Not Found");
-            echo "Ticket not found";
-            return;
-        }
-
-        // Check visibility access — return 404 rather than 403 to avoid leaking ticket existence
-        if (!$this->ticketModel->canUserAccessTicket($ticket, $currentUser)) {
-            header("HTTP/1.0 404 Not Found");
-            echo "Ticket not found";
+        if (!$ticket || !$this->ticketModel->canUserAccessTicket($ticket, $currentUser)) {
+            http_response_code(404);
+            include dirname(__DIR__) . '/views/error_404.php';
             return;
         }