hassio: add Home Assistant add-on support

[stkang]

Summary

This PR packages rethink-cloud as a native Home Assistant add-on, so it can
be installed from the Add-on Store, configured from the UI, and managed through
the Supervisor — no manual config file editing or separate container required.

Running as an add-on is opt-in and fully backward compatible: the existing
config.json flow is untouched. The add-on path only activates when the Home
Assistant Supervisor's options file (/data/options.json) is present.

What's included

• hassio/config.yaml — Add-on manifest: identity, supported architectures
  (aarch64, amd64, armv7), device-facing ports, Ingress web UI panel,
  the mqtt:need service dependency, and the user-configurable options schema.
• hassio/DOCS.md — End-user documentation shown on the add-on page:
  how it works, requirements, installation, options reference, ports, and
  data/persistence notes.
• repository.yaml — Add-on repository descriptor so the repo can be added
  to Home Assistant as a custom add-on source.
• util/hassio.ts — Builds a RawConfig from the add-on options
  (/data/options.json), with sensible defaults and MQTT broker resolution.
• tests/util/hassio.test.ts — Unit tests covering option parsing, defaults,
  and MQTT resolution.
• rethink-cloud.ts — Detects add-on mode via the options file and loads the
  config from options instead of config.json; the file-based path is unchanged.

How it works

1. The Supervisor writes user options to /data/options.json and mounts the
   persistent data volume.
2. On startup, rethink-cloud checks for that file. If present, it calls
   loadHassioConfig() to build the config from options; otherwise it reads
   config.json as before.
3. MQTT resolution: an explicit mqtt_url wins; otherwise the add-on asks the
   Supervisor for the Home Assistant MQTT service (mqtt:need, auto-detected);
   otherwise it falls back to mqtt://localhost:1883.
4. The management web interface is exposed through Ingress, so it opens
   directly from the add-on page with no extra port or login.

Configuration

All options are exposed under the add-on's Configuration tab and validated
against the schema in config.yaml. Key options: hostname, discovery_prefix,
rethink_prefix, bridge (ThinQ cloud bridge mode), mqtt_url / mqtt_user /
mqtt_pass, and log. See hassio/DOCS.md for the full reference.

Testing

• tests/util/hassio.test.ts covers option parsing, default fallbacks, and MQTT
  broker resolution (explicit URL, Supervisor service, localhost fallback).
• Backward compatibility: with no /data/options.json, the existing
  config.json flow runs unchanged.

Notes

• Fully backward compatible — non-add-on usage is unaffected.
• LG ThinQ is a trademark of LG, used for identification only. This project is
  not affiliated with LG and is provided WITHOUT ANY WARRANTY.

[anszom]

Thank you for the PR. I've just returned from vacation and I'm slowly working through the PR & issue backlog. I'd appreciate your patience :)