fix: remove CSP-blocked inline event handlers (onerror, onclick)

- Remove all onerror="this.style.display='none'" from avatar imgs in
  layout_header.php, DashboardView.php, and TicketView.php (PHP + JS)
- Replace onclick SLA dismiss with data-action="dismiss-priority-banner"
  attribute; handler wired via existing click delegation in TicketView.php
- Global capture-phase error delegation in layout_footer.php handles all
  avatar image failures by adding .lt-avatar-img-err class (CSS display:none)

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

views/DashboardView.php

@@ -331,7 +331,7 @@ include __DIR__ . '/layout_header.php';
       <div class="workload-item">
         <div class="lt-avatar lt-avatar--sm <?= $avatarColor ?>" aria-hidden="true" title="<?= htmlspecialchars($name) ?>">
           <?php if ($userId > 0): ?>
-            <img src="/api/user_avatar.php?user_id=<?= $userId ?>" alt="" class="lt-avatar-img" onerror="this.style.display='none'">
+            <img src="/api/user_avatar.php?user_id=<?= $userId ?>" alt="" class="lt-avatar-img">
           <?php endif ?>
           <span class="lt-avatar-initials"><?= htmlspecialchars($initials) ?></span>
         </div>