Fix header overlap, is-hidden missing globally, and CreateTicketView CSS

- base.css: add .lt-main.lt-container combined selector (specificity 0,2,0)
  to prevent responsive .lt-container padding shorthand from overriding
  the fixed-header clearance padding-top — affected all viewports < 1280px
- base.css: add .is-hidden { display: none !important } globally; it was
  only defined in ticket.css so dashboard ticketPreview popup rendered
  as a green box at 0,0 on page load instead of being hidden
- CreateTicketView.php: add dashboard.css to pageStyles so create-ticket-
  meta-grid, lt-form-hint, visibility-groups-list, duplicate-list classes
  are available (they were undefined when only ticket.css was loaded)

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

views/CreateTicketView.php

@@ -10,7 +10,7 @@ require_once __DIR__ . '/../middleware/CsrfMiddleware.php';
 $nonce       = SecurityHeadersMiddleware::getNonce();
 $pageTitle   = 'New Ticket';
 $activeNav   = 'dashboard';
-$pageStyles  = ['/assets/css/ticket.css?v=20260327'];
+$pageStyles  = ['/assets/css/dashboard.css?v=20260327', '/assets/css/ticket.css?v=20260327'];
 $pageScripts = [
     '/assets/js/keyboard-shortcuts.js?v=20260327',
 ];