fix: description unreadable in dark mode / OLED — swap disabled textarea for lt-markdown div

Root cause: disabled textarea gets opacity:0.45 + color:var(--text-muted) from
base.css, making it near-invisible on OLED (true-black background).

Fix:
- TicketView: add #ticketDescriptionView (div.lt-markdown) alongside the textarea;
  textarea is now hidden by default (style="display:none"), view div is shown
- ticket.js: renderDescriptionView() renders raw text via parseMarkdown() or nl2br;
  showDescriptionView() / showDescriptionEdit() swap between them;
  toggleEditMode() calls showDescriptionEdit() when entering edit, and
  renderDescriptionView() + showDescriptionView() when returning to read mode
- ticket.css: .ticket-description-view sets full-contrast text-primary/secondary
  colors, min-height, and line-height for comfortable reading

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

assets/css/ticket.css

@@ -269,3 +269,18 @@ kbd {
   .thread-depth-2 { margin-left: 1.5rem; }
   .thread-depth-3 { margin-left: 2.25rem; }
 }
+
+/* ── Description read view ───────────────────────────────────── */
+/* Shown in read mode instead of a disabled (faded) textarea.     */
+/* Uses lt-markdown typography for full contrast on dark/OLED.    */
+.ticket-description-view {
+  min-height: 8rem;
+  padding: 0.5rem 0.25rem;
+  line-height: 1.75;
+  color: var(--text-primary);
+}
+.ticket-description-view p {
+  color: var(--text-secondary);
+  margin-bottom: 0.6rem;
+}
+.ticket-description-view p:last-child { margin-bottom: 0; }