Add "OFF" mode like in the Hilo/Sinope apps

**Is your feature request related to a problem? Please describe.**

Many Sinope thermostats (including older models such as the TH1120 series) support an **"OFF" mode** in the official Sinope and Hilo apps. However, the Home Assistant “Sinope Local” integration exposes only limited `hvac_modes` (e.g., `heat`) and does **not allow switching the thermostat to OFF**. This means that a key device function is unavailable in Home Assistant even though it works locally through the hub.

The first automation I want to do with this is create a mapping of door sensors to baseboard heaters that will turn them off when a window/door is open

**Describe the solution you'd like**
Add support for the **OFF hvac_mode** for Sinope thermostats via the local API. Ideally:
- Expose `hvac_modes: [heat, off]` where supported
- Allow sending the command to set the thermostat to OFF
- Preserve and restore the previous temperature when switching back to HEAT (same behavior as the Sinope/Hilo app)

**Describe alternatives you've considered**
- Using the temperature setpoint workaround (setting very low temperatures), but this is not the same as OFF and still engages heating logic.

**Additional context**
- Sinope and Hilo apps can toggle OFF natively on the same devices.
- Devices are bound to the **Sinope hub**, so this integration is the only local path.
- Adding OFF would bring feature parity with the official apps and improve automation flexibility.