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/i18n/en.ts

@@ -219,6 +219,9 @@ export const en: Dict = {
   'export.customLabel': 'Longest edge (px)',
   'export.custom': 'EXPORT',
   'export.cap': 'This account can export up to {px}px.',
+  'export.share': 'Share to',
+  'export.shareHint':
+    "On a phone the network's own app opens with the picture already in its composer. On a desktop the picture goes to the clipboard — paste it (Ctrl+V) into the composer that opened; where pasting is refused, the file downloads for you to drop in.",
 
   'photo.uploading': 'UPLOADING…',