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/coordinates-display.css

@@ -0,0 +1,47 @@
+/* coordinates-display */
+
+#coordinates-display {
+  position: absolute;
+  top: 20px;
+  right: 250px;
+  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;
+  min-width: 180px;
+  backdrop-filter: blur(5px);
+}
+
+#coordinates-display .coord-item {
+  margin-bottom: 5px;
+  display: flex;
+  justify-content: space-between;
+}
+
+#coordinates-display .coord-label {
+  color: #aaa;
+}
+
+#coordinates-display .coord-value {
+  color: #4db8ff;
+  font-weight: 500;
+}
+
+#coordinates-display #zoom-level {
+  margin-top: 5px;
+  color: #ffff44;
+  font-weight: 500;
+  text-align: center;
+  font-size: 1rem;
+}
+
+#coordinates-display .mouse-coords {
+  font-size: 0.8rem;
+  color: #aaa;
+  margin-top: 5px;
+  padding-top: 5px;
+  border-top: 1px solid rgba(255, 255, 255, 0.1);
+}