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/css/base.css

@@ -0,0 +1,149 @@
+/* base.css - 公共基础样式 */
+
+* {
+  margin: 0;
+  padding: 0;
+  box-sizing: border-box;
+}
+
+body {
+  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
+  background-color: #0a0a1a;
+  color: #fff;
+  overflow: hidden;
+}
+
+#container {
+  position: relative;
+  width: 100vw;
+  height: 100vh;
+  /* user-select: none;
+  -webkit-user-select: none; */
+}
+
+#container.dragging {
+  cursor: grabbing;
+}
+
+#loading {
+  position: absolute;
+  top: 50%;
+  left: 50%;
+  transform: translate(-50%, -50%);
+  font-size: 1.2rem;
+  color: #4db8ff;
+  z-index: 100;
+  text-align: center;
+  background-color: rgba(10, 10, 30, 0.95);
+  padding: 30px;
+  border-radius: 10px;
+  border: 1px solid #4db8ff;
+  box-shadow: 0 0 30px rgba(77,184,255,0.3);
+}
+
+#loading-spinner {
+  border: 4px solid rgba(77, 184, 255, 0.3);
+  border-top: 4px solid #4db8ff;
+  border-radius: 50%;
+  width: 40px;
+  height: 40px;
+  animation: spin 1s linear infinite;
+  margin: 0 auto 15px;
+}
+
+@keyframes spin {
+  0% { transform: rotate(0deg); }
+  100% { transform: rotate(360deg); }
+}
+
+.error-message {
+  color: #ff4444;
+  margin-top: 10px;
+  font-size: 0.9rem;
+  display: none;
+  padding: 10px;
+  background-color: rgba(255, 68, 68, 0.1);
+  border-radius: 5px;
+  border-left: 3px solid #ff4444;
+}
+
+.terrain-controls {
+  margin-top: 15px;
+  padding-top: 15px;
+  border-top: 1px solid rgba(255, 255, 255, 0.1);
+}
+
+.slider-container {
+  margin-bottom: 10px;
+}
+
+.slider-label {
+  display: flex;
+  justify-content: space-between;
+  margin-bottom: 5px;
+  font-size: 0.9rem;
+}
+
+input[type="range"] {
+  width: 100%;
+  height: 8px;
+  -webkit-appearance: none;
+  background: rgba(0, 102, 204, 0.3);
+  border-radius: 4px;
+  outline: none;
+}
+
+input[type="range"]::-webkit-slider-thumb {
+  -webkit-appearance: none;
+  width: 16px;
+  height: 16px;
+  border-radius: 50%;
+  background: #4db8ff;
+  cursor: pointer;
+  box-shadow: 0 0 10px #4db8ff;
+}
+
+.status-message {
+  position: absolute;
+  top: 20px;
+  right: 20px;
+  background-color: rgba(10, 10, 30, 0.85);
+  border-radius: 10px;
+  padding: 10px 15px;
+  z-index: 10;
+  box-shadow: 0 0 20px rgba(0, 150, 255, 0.3);
+  border: 1px solid rgba(0, 150, 255, 0.2);
+  font-size: 0.9rem;
+  display: none;
+  backdrop-filter: blur(5px);
+}
+
+.status-message.success {
+  color: #44ff44;
+  border-left: 3px solid #44ff44;
+}
+
+.status-message.warning {
+  color: #ffff44;
+  border-left: 3px solid #ffff44;
+}
+
+.status-message.error {
+  color: #ff4444;
+  border-left: 3px solid #ff4444;
+}
+
+.tooltip {
+  position: absolute;
+  background-color: rgba(10, 10, 30, 0.95);
+  border: 1px solid #4db8ff;
+  border-radius: 5px;
+  padding: 5px 10px;
+  font-size: 0.8rem;
+  color: #fff;
+  pointer-events: none;
+  z-index: 100;
+  box-shadow: 0 0 10px rgba(77, 184, 255, 0.3);
+  display: none;
+  user-select: none;
+}