0.37.0: download whole folders (recursive) as a .zip from the file browser

The file browser/editor could only download individual files. Add a
recursive directory download that streams the folder as a zip archive,
on the local host and on every remote agent.

- file_service.archive_dir(): zip a directory recursively into a temp
  file, preserving the folder name as the archive root and empty
  subdirectories; symlinks are skipped (no sandbox escape / loops).
- /api/files/download and /agent/files/download branch on directories
  and return application/zip, cleaning up the temp file afterwards.
- Files page: show the download button for folders too (as <name>.zip).

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.36.1",
+  "version": "0.37.0",
   "type": "module",
   "scripts": {
     "dev": "vite",

frontend/src/pages/Files.tsx

@@ -202,7 +202,7 @@ export function Files() {
 
   const download = (e: HostPathEntry) =>
     filesApi
-      .download(join(path, e.name), e.name, host)
+      .download(join(path, e.name), e.type === "dir" ? `${e.name}.zip` : e.name, host)
       .catch((err) => toast.error(apiErrorMessage(err)));
 
   return (
@@ -407,15 +407,13 @@ export function Files() {
                     </td>
                     <td className="px-4 py-2">
                       <div className="flex items-center justify-end gap-1">
-                        {e.type === "file" && (
-                          <button
-                            title="Download"
-                            onClick={() => download(e)}
-                            className="rounded p-1.5 hover:bg-slate-100 dark:hover:bg-slate-700"
-                          >
-                            <Download className="h-4 w-4 text-slate-500" />
-                          </button>
-                        )}
+                        <button
+                          title={e.type === "dir" ? "Download as .zip" : "Download"}
+                          onClick={() => download(e)}
+                          className="rounded p-1.5 hover:bg-slate-100 dark:hover:bg-slate-700"
+                        >
+                          <Download className="h-4 w-4 text-slate-500" />
+                        </button>
                         {isAdmin && (
                           <>
                             <button