refctor: TankInfo structure (consistent layout)

- fix: use tagged enum serialization for TankError
- fix: rename TankInfo fields for consistent naming (volume_ml, pct, water_temp_c)
- renamed some fields for better clarity on contained value

rust/src/main.rs

@@ -310,7 +310,7 @@ async fn safe_main(spawner: Spawner) -> FatResult<()> {
         if let Some(err) = tank_state.got_error(&board.board_hal.get_config().tank) {
             match err {
                 TankError::SensorDisabled => { /* unreachable */ }
-                TankError::SensorMissing(raw_value_mv) => log(
+                TankError::SensorMissing { raw_mv: raw_value_mv } => log(
                     LogMessage::TankSensorMissing,
                     raw_value_mv as u32,
                     0,
@@ -324,8 +324,8 @@ async fn safe_main(spawner: Spawner) -> FatResult<()> {
                     &format!("{value}"),
                     "",
                 ),
-                TankError::BoardError(err) => {
-                    log(LogMessage::TankSensorBoardError, 0, 0, "", &err.to_string())
+                TankError::BoardError { message: err } => {
+                    log(LogMessage::TankSensorBoardError, 0, 0, "", &err)
                 }
             }
             // disabled cannot trigger this because of wrapping if is_enabled