Fix NaN target temp for native Daikin splits

Daikin's local API returns stemp as the placeholder string '-' in
some states, which Number() turns into NaN instead of null/undefined.
That NaN wasn't caught by the downstream ?? null checks, so it got
written into current_target_temp and device_readings.target_temp as
a literal NaN, then rendered as "NaN°C" on the dashboard and zone
popup. Guard stemp the same way roomTemp's htemp is already guarded.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/lib/drivers/daikin.js

@@ -187,7 +187,7 @@ export async function getStatus(externalRef) {
       external_ref: externalRef,
       onOff: control.pow === '1' ? 'on' : 'off',
       mode: DAIKIN_TO_MODE[control.mode] ?? 'auto',
-      setpoint: control.stemp != null ? Number(control.stemp) : undefined,
+      setpoint: control.stemp != null && control.stemp !== '-' ? Number(control.stemp) : undefined,
       fanSpeed: DAIKIN_TO_FAN[control.f_rate] ?? 'auto',
       roomTemp: sensor.htemp != null && sensor.htemp !== '-' ? Number(sensor.htemp) : undefined,
     }