0.37.1: fix folder-download hang/501 on special files + add download progress

Two issues with the 0.37.0 folder zip-download:

1. Hang / server error (reported as 501) on "some folders". archive_dir
   tried to zip every entry, including non-regular files. Opening a FIFO
   blocks forever (no writer); a unix socket / unreadable file raised an
   OSError that aborted the whole archive. Now only real regular files are
   zipped — FIFOs, sockets, devices and symlinks are skipped without ever
   open()-ing them, and a per-file read error skips just that file instead
   of failing the download.

2. No feedback while a large folder is being prepared. The zip is built
   server-side before any bytes flow, so the click felt dead. The Files
   page now shows an indeterminate "Preparing <name>…" bar from click,
   switching to a real percentage during the transfer (Content-Length is
   known for the finished zip). filesApi.download forwards onDownloadProgress.

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.37.0",
+  "version": "0.37.1",
   "type": "module",
   "scripts": {
     "dev": "vite",