Fix close-ticket UX, add cmd palette hint, breadcrumb, image lightbox

- ticket.js: status change requiring a comment now shows an inline
  modal with a textarea — comment is actually posted before the status
  changes, instead of just warning the user and changing anyway
- layout_header.php: add ⌘K button in header so users can discover
  the command palette; also removes inline onclick in favor of JS
  (CSP-safe via nonce script block already present)
- TicketView.php: upgrade breadcrumb to lt-breadcrumb markup with
  ticket title preview (truncated at 45 chars) and aria-current
- ticket.js + ticket.css: image attachments now render as clickable
  thumbnails (3rem×3rem) that open in lt.lightbox; non-image files
  keep the icon display unchanged

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

assets/css/ticket.css

@@ -195,6 +195,19 @@ body.edit-mode .editable-metadata {
   gap: 0.4rem;
 }
 
+/* Image thumbnail in attachment list */
+.attachment-thumb {
+  display: block;
+  width: 3rem;
+  height: 3rem;
+  object-fit: cover;
+  border-radius: 3px;
+  border: 1px solid var(--border-color);
+  cursor: zoom-in;
+  flex-shrink: 0;
+}
+.lt-lightbox-trigger { display: block; line-height: 0; }
+
 /* ── Dependencies list ───────────────────────────────────────── */
 .dependencies-list {
   display: flex;