Description editor: preserve newlines on export, toggle Quote, highlight active toolbar buttons

- Enter now starts a new <p> (defaultParagraphSeparator) so line breaks
  in the description box show up as line breaks in HTML/Markdown/Confluence
  exports; htmlToMarkdown also handles <div>-separated paragraphs.
- Clicking Quote while already in a blockquote toggles back to a paragraph.
- Toolbar buttons (Bold, Italic, Bullet, Number, Quote) turn blue to
  reflect the formatting state at the current cursor/selection.

app/renderer/style.css

@@ -487,6 +487,11 @@ kbd {
   margin-bottom: 8px;
 }
 .rich-toolbar button { padding: 4px 8px; font-size: 12px; }
+.rich-toolbar button.active {
+  background: var(--accent);
+  border-color: var(--accent);
+  color: var(--accent-fg);
+}
 .rich-editor {
   min-height: 110px;
   max-height: 220px;