Add video picker and Discord video attachments.

The picker accepts one MP4 or WebM, the thread lightbox zooms photos, and Discord gets the video as a file on a follow-up message so photo embeds still render.

templates/base.html

@@ -73,6 +73,13 @@
   <footer class="site-footer">
     <p>This site is not a substitute for a licensed plumber. Advice is general and based on the question as written. If you have a gas leak, flooding, or another emergency, leave the area if needed and call 911.</p>
   </footer>
+  <dialog class="image-zoom" data-image-zoom-dialog aria-labelledby="image-zoom-caption">
+    <form method="dialog" class="image-zoom-bar">
+      <button class="image-zoom-close" value="close">Close</button>
+    </form>
+    <img data-image-zoom-img alt="">
+    <p id="image-zoom-caption" class="image-zoom-caption" data-image-zoom-caption></p>
+  </dialog>
 </body>
 </html>
 {{end}}