scrypted: mdns discovery — browse, host choices, auto-heal

Browse for _scrypted-viewport._tcp with a plain dgram socket on an
EPHEMERAL port: per RFC 6762 §6.7 a query from a non-5353 source port is
a legacy unicast query and responders reply unicast to that port —
verified in the ESP-IDF responder (mdns_send.c answers to the querier's
addr/port whenever src_port != 5353). No :5353 bind means no conflict
with Scrypted's own HomeKit mDNS or a host avahi-daemon, and it works
under Docker host-networking or native alike. One response packet
carries PTR+SRV+TXT+A (RFC 6763 §12.1), so parsing is per-packet with
no follow-up queries; the wire codec (name compression included) is
~150 lines, no dependencies.

Surfaced three ways:
- add-device form: host field is a combobox listing discovered
  viewports as "ip — name (vX, WxH)"; manual entry still works and
  only the address token is stored.
- child settings page: same choices on the existing host field.
- auto-heal: when a /config register fails, re-browse and match by MAC
  (seeded from /state and discovery; survives renames) then by name;
  on a hit at a new address, rewrite host and retry once. Runs only on
  register failure, so it's rate-limited to the 5-min cycle and removes
  the need for a DHCP reservation.

Browses are best-effort ([] on any failure) and cached 30s so settings
re-renders don't spam the LAN. diagnostic.ts gains the same browse as a
paste-and-run probe that validates dgram + multicast reachability from
inside the real Scrypted sandbox before trusting the feature.

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