feat(files): preview attachments via short-lived signed URLs

Downloads require an Authorization header, which <iframe>/<img> cannot
send. The previous blob: workaround broke under download managers (IDM
hooks blob URLs, fails to resolve them, and leaves a 0-byte file) and in
Chrome's mobile emulation, which has no inline PDF plugin.

Backend:
- preview_token.py: HMAC-signed token bound to one (quote, kind, file)
  with a 5-minute TTL, keyed off an existing secret
- GET /quotes/{id}/preview-url/{kind}/{attachment} mints the URL
- the two file GET routes accept a Bearer session OR the signed ?t= token
  (they leave the global auth dep, so the other six file routes now carry
  it explicitly — nothing is open by omission)

Frontend:
- api.files.previewUrl() replaces the blob helper
- shared FilePreviewModal + useFilePreview; FileList and the dashboard
  quote card both use it
- PDFs render with the browser's native viewer on desktop and pdf.js on
  canvas where no inline viewer exists (mobile); pdf.js is fed the bytes
  in memory so no PDF URL is exposed for a download manager to intercept
- removed the per-attachment download buttons on the dashboard card and
  the modal header

Ops:
- nginx: explicit MIME types (.mjs as text/javascript) — a module worker
  served as octet-stream is refused and crashed the page
- frontend service bypasses the nginx entrypoint, whose apk manifest call
  hangs when the package mirror is unreachable (502s)

docker-compose.yml

@@ -289,6 +289,11 @@ services:
         VITE_OPENPANEL_CLIENT_ID: ${OPENPANEL_CLIENT_ID:-}
         VITE_SENTRY_DSN: ${SENTRY_DSN_FRONTEND:-}
     container_name: procurement-frontend
+    # از entrypoint پیش‌فرض image nginx رد می‌شویم: آن اسکریپت `apk manifest`
+    # اجرا می‌کند و اگر دسترسی به مخزن بسته‌ها نباشد بی‌نهایت معلق می‌ماند
+    # (سرویس بالا نمی‌آید و traefik خطای 502 می‌دهد). کانفیگ ما مستقل است و
+    # نیازی به آن اسکریپت‌ها ندارد.
+    entrypoint: ["nginx", "-g", "daemon off;"]
 
     labels:
       - traefik.enable=true