Fix upload jumping straight to camera on mobile

The file input's capture="environment" attribute forces mobile browsers
to open the camera directly, skipping the OS file picker entirely -- so
there was no way to pick an existing photo from the phone, only take a
new one. Dropping capture restores the normal camera-or-library chooser
on mobile; desktop drag-and-drop is unaffected.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

index.html

@@ -19,7 +19,7 @@
         <span class="drop-zone-label">choose a photo, or drag one here</span>
         <span class="drop-zone-sub">nothing leaves your browser until you say so</span>
       </label>
-      <input type="file" id="file-input" accept="image/*" capture="environment" hidden>
+      <input type="file" id="file-input" accept="image/*" hidden>
     </div>
 
     <div class="card" id="editor-card" hidden>