Add Elfin EW11 restart recovery button

[mel9320107]

What changed

• Add an Elfin EW11 management helper that reproduces the web UI restart request (POST /cmd, CID 20003, empty payload).
• Add a separate Restart Elfin bridge diagnostic button; this is distinct from Restart stream, which only rebuilds Home Assistant's TCP connection.
• Keep the bridge restart button available when the spa status stream is unavailable and disable it by default for installations using non-Elfin bridges.
• Treat a server disconnect during the restart request as expected because the EW11 may reboot before completing its HTTP response.
• Document the recovery path for the failure mode observed on hardware: EW11 network/web UI and TCP listener remain alive, but port 4257 emits invalid data until the adapter is restarted.

Hardware validation

The underlying request was reverse-engineered directly from the installed EW11 web UI and tested manually against the hardware. Sending:

msg={"CID":20003,"PL":{}}

with HTTP Basic authentication to /cmd successfully restarted the adapter and restored valid 0x7E-framed spa traffic.

Current limitation

The helper currently uses the stock EW11 HTTP Basic credentials admin / admin. The button is disabled by default. If configurable management credentials are needed later, they should be added through the config/options flow rather than embedding installation-specific values.