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>

backend/services/file_service.py

@@ -187,7 +187,13 @@ def archive_dir(path: str) -> tuple[str, str]:
 
     Returns ``(tmp_zip_path, download_filename)``. The caller is responsible
     for deleting the temp file once it has been streamed to the client.
-    Symlinks are skipped so the archive cannot escape the sandbox or loop.
+
+    Only regular files and real subdirectories are archived. Symlinks are
+    skipped (no sandbox escape / loops); special files (FIFOs, sockets,
+    devices) are skipped too — opening a FIFO would block forever and a
+    socket can't be read at all. Files that can't be read (permissions, or
+    that vanish mid-walk) are skipped individually rather than aborting the
+    whole archive.
     """
     real = _safe_real(path)
     if not os.path.isdir(real):
@@ -207,10 +213,18 @@ def archive_dir(path: str) -> tuple[str, str]:
                     zf.writestr(os.path.join(name, rel_root) + "/", "")
                 for f in files:
                     full = os.path.join(root, f)
-                    if os.path.islink(full):
+                    # os.path.isfile follows symlinks; combined with the islink
+                    # check it admits only real regular files (skips FIFOs,
+                    # sockets, devices and symlinks without ever open()-ing them).
+                    if os.path.islink(full) or not os.path.isfile(full):
+                        continue
+                    arc = (os.path.join(name, rel_root, f) if rel_root != "."
+                           else os.path.join(name, f))
+                    try:
+                        zf.write(full, arc)
+                    except OSError:
+                        # Unreadable or vanished mid-walk — skip just this file.
                         continue
-                    zf.write(full, os.path.join(name, rel_root, f) if rel_root != "."
-                             else os.path.join(name, f))
     except OSError:
         if os.path.exists(tmp):
             os.unlink(tmp)