feat(studio): hand an export straight to a social network

EXPORT only ever wrote a file, and posting it meant leaving the studio, finding
the file and uploading it again. The dialog now offers FACEBOOK, INSTAGRAM,
REDDIT and X beside the sizes, and the render goes to the network instead of the
download folder.

What the browser can do decides how, since a page may not touch another origin's
composer: a phone's share sheet opens the target app with the picture already
attached, a desktop gets the picture on the clipboard and the site in a new tab
for one paste, and an origin with no clipboard API downloads the file beside the
open site. The line under the row says so before the click.

docker/frontend/src/styles/app.css

@@ -1046,6 +1046,28 @@ button.hint:disabled { cursor: default; }
 }
 .export-custom .field { width: 96px; }
 
+/* Hand the export to a network. A hairline over it, so the row reads as the
+   second way out of the dialog and not as a sixth size. */
+.export-share {
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+  margin-top: 12px;
+  padding-top: 12px;
+  border-top: 1px solid var(--border);
+  text-align: left;
+}
+.export-share-row {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 8px;
+}
+.export-share-row .btn {
+  flex: 1 1 auto;
+  font-size: 12px;
+  letter-spacing: 0.06em;
+}
+
 /* --- strip moderation (/admin) ------------------------------------------ */
 .adm { height: 100%; overflow-y: auto; padding: 0 22px 40px; }
 /* Sticky: the mark and the page name stay in the top-left corner while the