0.37.2: stream folder zip-downloads to fix 504 on large folders

A big folder hit a 504 Gateway Timeout: the zip was built into a temp
file *before* any response was sent, so for large folders the backend
stayed silent past nginx's proxy_read_timeout.

Now the zip is streamed as it's built, end to end:
- file_service.open_archive() returns (filename, byte iterator); _iter_zip
  walks the dir and yields zip bytes incrementally via a small drain
  buffer, writing each file in 1 MiB chunks (bounded memory, valid CRCs).
  Same hardening as before — only real regular files; FIFOs/sockets/
  devices/symlinks skipped without open(); per-file read errors skipped.
- /api/files/download and /agent/files/download return a StreamingResponse
  (no temp file). The agent proxy streams the agent response straight
  through (agent_service.stream_download), pulling the first chunk eagerly
  so an offline/bad-token agent still yields a clean status before 200.
- Files page: streamed downloads have no Content-Length, so the progress
  bar shows the running downloaded byte count ("Downloading … 12.3 MB")
  instead of a percentage, after the initial "Preparing …".

Verified end to end via TestClient (200, application/zip, valid zip,
2 MiB file intact, FIFO skipped, no hang).

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.37.1",
+  "version": "0.37.2",
   "type": "module",
   "scripts": {
     "dev": "vite",

frontend/src/pages/Files.tsx

@@ -88,6 +88,7 @@ export function Files() {
     pct: number;
     kind: "upload" | "download";
     indeterminate?: boolean;
+    loaded?: number;
   } | null>(null);
   const fileInput = useRef<HTMLInputElement>(null);
   const folderInput = useRef<HTMLInputElement>(null);
@@ -222,6 +223,7 @@ export function Files() {
             pct: total ? (loaded / total) * 100 : 0,
             kind: "download",
             indeterminate: !total,
+            loaded,
           }),
       )
       .catch((err) => toast.error(apiErrorMessage(err)))
@@ -329,14 +331,18 @@ export function Files() {
               )}
               <span className="truncate">
                 {progress.kind === "download"
-                  ? progress.indeterminate
+                  ? progress.indeterminate && !progress.loaded
                     ? `Preparing ${progress.label}…`
                     : `Downloading ${progress.label}`
                   : `Uploading ${progress.label}`}
               </span>
             </span>
             <span className="tabular-nums">
-              {progress.indeterminate ? "" : `${Math.round(progress.pct)}%`}
+              {progress.indeterminate
+                ? progress.loaded
+                  ? formatBytes(progress.loaded)
+                  : ""
+                : `${Math.round(progress.pct)}%`}
             </span>
           </div>
           <div className="h-2 w-full overflow-hidden rounded-full bg-slate-200 dark:bg-slate-700">