feat(earth): toolbar and zoom improvements

- Add box-sizing/padding normalization to toolbar buttons
- Remove zoom slider, implement click/hold zoom behavior (+/- buttons)
- Add 10% step on click, 1% continuous on hold
- Fix satellite init: show satellite points immediately, delay trail visibility
- Fix breathing effect: faster pulse, wider opacity range
- Add toggle-cables functionality with visibility state
- Initialize satellites and cables as visible by default

frontend/public/earth/js/ui.js

@@ -25,7 +25,8 @@ export function updateZoomDisplay(zoomLevel, distance) {
   const percent = Math.round(zoomLevel * 100);
   document.getElementById('zoom-value').textContent = percent + '%';
   document.getElementById('zoom-level').textContent = '缩放: ' + percent + '%';
-  document.getElementById('zoom-slider').value = zoomLevel;
+  const slider = document.getElementById('zoom-slider');
+  if (slider) slider.value = zoomLevel;
   document.getElementById('camera-distance').textContent = distance + ' km';
 }