[piciuok]

DS18B20 sensors are only accurate to ±0.5 °C, and pipe-mounted sensors add their own error. That makes temperature differences between sensors (e.g. DHW outlet vs. circulation return) unreliable. This PR adds a per-sensor calibration offset in 0.1 °C steps (range −10.0…+10.0).

Changes

• Web UI: new "Offset (°C)" column in the 1-wire sensor table (number input, step 0.1, saved on change)
• HTTP: /dallasoffset?<address>=<value>
• MQTT: set via <base>/1wire/<address>/offset/set (accepts 0.3 and 0,3); state published (retained) on <base>/1wire/<address>/offset
• /json: new Offset field per sensor
• The offset is applied to the raw reading before the delta filter, so MQTT, web, JSON and rules (ds18b20#...) all see the corrected value. The 85.00 °C power-on-reset check still uses the raw value.
• Changing the offset shifts the last known value immediately and republishes it, so there is no wait for the next reading.
• Offsets are stored in /dallasoffset.json, separate from dallas.json, so a downgrade to older firmware keeps reading the aliases.
• Removing a sensor also clears its retained offset topic.
• README section added.

Testing

• Compiles for ESP8266 and ESP32-S3 with the CI FQBNs.
• Tested on a large (ESP32-S3) board with 3 DS18B20 sensors:
  • offsets set from the web UI survive a reboot
  • /json returns the Offset field
  • changing an offset (0.2 → 0.5) shifts the reported temperature immediately
  • the log shows 1wire sensor <address> offset set to <value>

Screenshots