Estimated Target Time Support and Performance Improvements

New Features

• Added support for the new Estimated Target Time API field.
• Added a new Estimated Target Time sensor that displays the remaining heating time while the device is active.
• The estimated time is also available as an attribute on the climate entity.
• Updated the Power Sensor to use modern Home Assistant sensor enums.

User Experience

• Improved the configuration flow by removing the default "Username" and "Password" placeholder values from the login form.

Performance & Reliability

• The integration now uses Home Assistant's shared HTTP session, improving performance through connection reuse and ensuring compliance with HA networking guidelines.
• Added a 10-second timeout to all API requests to prevent blocking during network issues.

Climate Entity Improvements

• Fixed turn-on and turn-off handling to fully support modern Home Assistant service calls.
• Fixed an issue where setting the HVAC mode to Off could unexpectedly turn the device back on when it was already off.
• Removed deprecated backward-compatibility flags.

Maintenance

• Existing dolphin.xxx entity IDs are preserved to avoid breaking dashboards and automations.
• Home Assistant may still display a deprecation warning related to entity naming. This warning can be safely ignored for now.