0.37.4: fix folder upload doing nothing (set webkitdirectory reliably)

"Upload folder" silently did nothing: the directory-selection attribute
was set on the hidden <input> via a JSX spread
({...{webkitdirectory:"", directory:""}}), which React doesn't reliably
apply to the DOM — and if isAdmin resolves after first render, a one-shot
effect would miss the input mounting entirely. Without the attribute the
picker is a plain file picker where no folder can be selected, so the user
picks nothing and nothing happens.

- Set webkitdirectory/directory/mozdirectory imperatively through a
  callback ref, which runs whenever the input mounts. folderInput is now a
  MutableRefObject so the callback can populate it.
- Folder upload now shows the progress bar immediately on start (small
  files can finish before the browser emits any upload-progress event, so
  don't wait for the first one to render feedback).

Frontend-only; all 3 images rebuilt+pushed 0.37.4.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.37.3",
+  "version": "0.37.4",
   "type": "module",
   "scripts": {
     "dev": "vite",