Fix #3: Preserve gas_daily_kwh sensor on reboot or no new data

When there is no new GRDF data or on reboot, the gas_daily_kwh sensor was disappearing.
Now it preserves the last known value by reading it from Home Assistant and updating the sensor.

This ensures the sensor remains available even when no new data is received.

Author: mistral-vibe

apps/meters_to_ha/meters_to_ha.py

@@ -3515,6 +3515,36 @@ def update_grdf_device(self, json_file):
                 update_state_file(
                     self.configuration[STATE_FILE], {"grdf": entity_data}
                 )
+            
+            # Preserve gas_daily_kwh sensor on reboot or no new data (issue #3)
+            # Try to get the last known value from HA
+            for daily_sensor in (
+                sensor_name_daily_pce_kwh,
+                sensor_name_daily_generic_kwh,
+            ):
+                try:
+                    response = self.open_url(HA_API_SENSOR_FORMAT % (daily_sensor,))
+                    if isinstance(response, dict) and "state" in response:
+                        # Preserve the last known value
+                        last_daily_kwh = response["state"]
+                        last_attributes = response.get("attributes", {})
+                        
+                        # Update the sensor to preserve its value
+                        preserve_data = {
+                            "state": last_daily_kwh,
+                            "attributes": {
+                                **last_attributes,
+                                "last_check": now_isostr,
+                            },
+                        }
+                        r = self.open_url(
+                            HA_API_SENSOR_FORMAT % (daily_sensor,), preserve_data
+                        )
+                        self.mylog(f"Preserved {daily_sensor}: {last_daily_kwh}")
+                        break
+                except RuntimeError:
+                    # Sensor doesn't exist yet or is unavailable
+                    pass
         else:
             self.mylog(
                 f"    update value is {date_time.isoformat()}:"