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/tailwind.config.ts

@@ -37,6 +37,16 @@ export default {
         pill: "var(--sp-r-pill)",
         chip: "var(--sp-r-chip)",
       },
+      keyframes: {
+        // Sliding bar for indeterminate progress (e.g. server zipping a folder).
+        indeterminate: {
+          "0%": { transform: "translateX(-100%)" },
+          "100%": { transform: "translateX(400%)" },
+        },
+      },
+      animation: {
+        indeterminate: "indeterminate 1.2s ease-in-out infinite",
+      },
     },
   },
   plugins: [],