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/layout_header.php

@@ -166,7 +166,6 @@ $_lt_assetVer  = $GLOBALS['config']['ASSET_VERSION'] ?? '20260329';
           <img src="/api/user_avatar.php?user_id=<?= $_lt_userId ?>"
                alt=""
                class="lt-avatar-img"
-               onerror="this.style.display='none'">
         <?php endif ?>
         <span class="lt-avatar-initials"><?= htmlspecialchars($_lt_initials) ?></span>
       </div>