clouds3d: closer cruise, sun-alt-driven stars

- Cruise altitude 2.6 -> 2.0 (just above cloud ceiling y=1.5,
  gives the 'just above the clouds' airliner feel)
- Expose __currentSunAlt (radians) from animate() so the
  renderer can tell night from day without misreading
  realDayProgress (which mixes pre-dawn + post-sunset hours)
- Replace (1 - uDayProgress) star gate by smoothstep over uSunAlt:
  nightFactor = 1 when sun is well below horizon, 0 when above
- Widen sky band so stars are visible from the horizon upward
  (cockpit-perspective frame is mostly below rd.y=0.3)

background.html

@@ -1657,6 +1657,9 @@ window.weatherThemes = {
             window.__currentWeatherName = currentWeatherThemeName;
             window.__currentDayProgress = realDayProgress;
             window.__currentThemeSpeed = currentThemeSpeed;
+            // Sun altitude in radians. Negative = below horizon (night).
+            var _sunPos = SunCalc.getPosition(new Date(), PASSAU_LAT, PASSAU_LON);
+            window.__currentSunAlt = _sunPos.altitude;
             effect.setOptions({
                 skyColor: c.sky,
                 cloudColor: c.cloud,