feat(browse): conversion links open in a new tab

Drops the `download` attribute and adds `target="_blank"`. Click now
opens the conversion URL in a new tab; the server's
Content-Disposition: inline routes each format to a sensible
behavior in that tab:

  • HTML → renders as a web page
  • PDF  → opens in the browser's PDF viewer
  • DOCX → auto-downloads (browser can't render Office Open XML),
           tab is transient

Right-click → "Save Link As" still works for explicit save-to-disk.

Side benefit for debugging: when the conversion endpoint returns
422 or 503, the response body appears as a plain-text page in the
new tab, which is easier to triage than a transient toast.

browse/js/preview-markdown.js

@@ -456,12 +456,21 @@
                 var a = document.createElement('a');
                 a.className = 'btn btn-sm btn-secondary md-shell__download';
                 a.href = node.url + '?convert=' + encodeURIComponent(fmt);
-                a.download = node.name.replace(/\.md$/i, '') + '.' + fmt;
+                // target=_blank: clicks open in a new tab. The server
-                a.textContent = fmt.toUpperCase();
+                // sends Content-Disposition: inline, so the new tab
-                a.title = 'Download as ' + fmt.toUpperCase()
+                // either renders (HTML → web page; PDF → browser's
-                    + '  (right-click to copy link or open in new tab)';
+                // PDF viewer) or auto-downloads (DOCX, since browsers
-                a.dataset.fmt = fmt;
+                // can't render Office Open XML). Right-click "Save
+                // Link As" still gives a download-to-disk path for
+                // any format. Errors from the server (422, 503, …)
+                // appear as a plain-text page in the new tab, which
+                // is more diagnostic than a transient toast.
+                a.target = '_blank';
                 a.rel = 'noopener';
+                a.textContent = fmt.toUpperCase();
+                a.title = 'Open ' + fmt.toUpperCase()
+                    + ' in a new tab  (right-click for Save Link As / Copy Link)';
+                a.dataset.fmt = fmt;
                 convertBtns.push(a);
             });
         }