TURN relay candidates were applied only when IS_MOBILE, leaving desktop
clients STUN-only. Customers behind symmetric NAT / corporate firewalls /
VPNs then couldn't establish the peer connection -> connectionState 'failed'
-> "connection lost" -> blank screen right after granting permissions. This
hit only some users (those needing a relay).

Apply the /api/ice config (STUN + managed TURN) regardless of device, in both
the customer (share.html) and agent (connect.html) flows. Requires TURN_URLS /
TURN_USERNAME / TURN_CREDENTIAL to be set in the production environment.

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