feat(earth): Modularize 3D Earth page with ES Modules

## Changelog

### New Features
- Modularized 3D earth HTML page from single 1918-line file into ES Modules
- Split CSS into separate module files (base, info-panel, coordinates-display, legend, earth-stats)
- Split JS into separate modules (constants, utils, ui, earth, cables, controls, main)

### 3D Earth Rendering
- Use Three.js r128 (via esm.sh CDN) for color consistency with original
- Earth with 8K satellite texture and proper material settings
- Cloud layer with transparency and additive blending
- Starfield background (8000 stars)
- Latitude/longitude grid lines that rotate with Earth

### Cable System
- Load cable data from geo.json with great circle path calculation
- Support for MultiLineString and LineString geometry types
- Cable color from geo.json properties.color field
- Landing points loading from landing-point-geo.geojson

### User Interactions
- Mouse hover: highlight cable and show details
- Mouse click: lock cable with pulsing glow effect
- Click cable to pause rotation, click elsewhere to resume
- Click rotation toggle button to resume rotation and clear highlight
- Reset view with smooth animation (800ms cubic ease-out)
- Mouse wheel zoom support
- Drag to rotate Earth

### UI/UX Improvements
- Tooltip shows latitude, longitude, and altitude
- Prevent tooltip text selection during drag
- Hide tooltip during drag operation
- Blue border tooltip styling matching original
- Cursor changes to grabbing during drag
- Front-facing cable detection (only detect cables facing camera)

### Bug Fixes
- Grid lines now rotate with Earth (added as Earth child)
- Reset view button now works correctly
- Fixed camera reference in reset view
- Fixed autoRotate state management when clearing locked cable

### Original HTML
- Copied original 3dearthmult.html to public folder for reference

frontend/public/earth/js/ui.js

@@ -0,0 +1,79 @@
+// ui.js - UI update functions
+
+// Show status message
+export function showStatusMessage(message, type = 'info') {
+  const statusEl = document.getElementById('status-message');
+  statusEl.textContent = message;
+  statusEl.className = `status-message ${type}`;
+  statusEl.style.display = 'block';
+  
+  setTimeout(() => {
+    statusEl.style.display = 'none';
+  }, 3000);
+}
+
+// Update coordinates display
+export function updateCoordinatesDisplay(lat, lon, alt = 0) {
+  document.getElementById('longitude-value').textContent = lon.toFixed(2) + '°';
+  document.getElementById('latitude-value').textContent = lat.toFixed(2) + '°';
+  document.getElementById('mouse-coords').textContent = 
+    `鼠标: ${lat.toFixed(2)}°, ${lon.toFixed(2)}°`;
+}
+
+// Update zoom display
+export function updateZoomDisplay(zoomLevel, distance) {
+  document.getElementById('zoom-value').textContent = zoomLevel.toFixed(1) + 'x';
+  document.getElementById('zoom-level').textContent = '缩放: ' + zoomLevel.toFixed(1) + 'x';
+  document.getElementById('zoom-slider').value = zoomLevel;
+  document.getElementById('camera-distance').textContent = distance + ' km';
+}
+
+// Update cable details
+export function updateCableDetails(cable) {
+  document.getElementById('cable-name').textContent = cable.name || 'Unknown';
+  document.getElementById('cable-owner').textContent = cable.owner || '-';
+  document.getElementById('cable-status').textContent = cable.status || '-';
+  document.getElementById('cable-length').textContent = cable.length || '-';
+  document.getElementById('cable-coords').textContent = cable.coords || '-';
+  document.getElementById('cable-rfs').textContent = cable.rfs || '-';
+}
+
+// Update earth stats
+export function updateEarthStats(stats) {
+  document.getElementById('cable-count').textContent = stats.cableCount || 0;
+  document.getElementById('landing-point-count').textContent = stats.landingPointCount || 0;
+  document.getElementById('terrain-status').textContent = stats.terrainOn ? '开启' : '关闭';
+  document.getElementById('texture-quality').textContent = stats.textureQuality || '8K 卫星图';
+}
+
+// Show/hide loading
+export function setLoading(loading) {
+  const loadingEl = document.getElementById('loading');
+  loadingEl.style.display = loading ? 'block' : 'none';
+}
+
+// Show tooltip
+export function showTooltip(x, y, content) {
+  const tooltip = document.getElementById('tooltip');
+  tooltip.innerHTML = content;
+  tooltip.style.left = x + 'px';
+  tooltip.style.top = y + 'px';
+  tooltip.style.display = 'block';
+}
+
+// Hide tooltip
+export function hideTooltip() {
+  document.getElementById('tooltip').style.display = 'none';
+}
+
+// Show error message
+export function showError(message) {
+  const errorEl = document.getElementById('error-message');
+  errorEl.textContent = message;
+  errorEl.style.display = 'block';
+}
+
+// Hide error message
+export function hideError() {
+  document.getElementById('error-message').style.display = 'none';
+}