fix(drop): stream every dropped file to disk; no local size cap

- POST /api/run/drop-file pipes the body to its file as it arrives in every
  case — a member's (S3) through `write-as … replace` with the bytes on
  stdin — instead of buffering up to 20 MB in memory for a remote browser.
- Remote limit (server mode, a LAN browser) 20 MB → 500 MB, as a Files upload.
- A loopback browser in local mode has no cap any more, only the data dir's
  free space less a 1 GB reserve; ETOOBIG's max_bytes is what still fits, so
  the UI's "at most N MB" stays true.
- drop-image keeps its 20 MB in-memory limit, now checked on its own.
- fake-sudo gains a spawn for run-as.__setSpawnImpl.

docs/SERVER-MODE.md

@@ -65,7 +65,7 @@ The proxy has to:
 - forward WebSocket upgrades (`/ws` for live updates, `/ws-pty/*` for terminals);
 - set `X-Forwarded-For` and `X-Forwarded-Proto`;
 - accept request bodies as large as AgentHub's own limits (Files uploads up to
-  500 MB, prompt drops 20 MB). nginx's default `client_max_body_size` is 1 MB,
+  500 MB, files dropped on a terminal or a prompt box 500 MB, dropped images 20 MB). nginx's default `client_max_body_size` is 1 MB,
   and its own 413 is an HTML page the UI can only report as a failed upload.
 
 nginx: