On a remote gateway connection, agent-written files live on the gateway
host, not the desktop's disk, so the Artifacts view's file:// hrefs failed
("Invalid external URL") and image thumbnails broke.
Make mediaExternalUrl() remote-aware in one place: in remote mode it
rewrites gateway-local paths to GET /api/files/download (a new endpoint
that streams the file as a Content-Disposition: attachment). The artifacts
view now resolves through it, and so do the existing chat-media and
generated-image callers, for free.
The download endpoint stays auth-gated; auth_middleware additionally
accepts the session token as a ?token= query param for this one path so a
shell/browser-opened download (which can't set the session header) still
authenticates — the same query-token tradeoff as the /api/pty WebSocket.
It is NOT added to PUBLIC_API_PATHS.
Salvages #46663 (which carried ~19k lines of CRLF noise and made the
endpoint public). Reimplemented on a clean LF base with the security hole
closed and tests added.
Co-authored-by: qingshan89 <qs2816661685@gmail.com>
