#2 CoTurn Server Setup

- /api/ice: when TURN_SECRET is set, mint short-lived HMAC credentials
  (coturn use-auth-secret) so no permanent password is exposed and the relay
  can't be abused. Static TURN_USERNAME/CREDENTIAL still supported.
- share.html: connection watchdog + clear "couldn't connect on this network"
  message instead of a blank screen when no path can be established.
- deploy/coturn: ready-to-run turnserver.conf + docker-compose + README for
  hosting our own TURN on a VM we own (flat cost, no per-GB billing).

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

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>