feat: holographic Earth slowly rotating above the Nexus

- Procedural holographic Earth using simplex-noise continent shader
  with scan lines, Fresnel rim glow, lat/lon grid overlay and
  atmosphere shell (all pre-existing in main)
- Add gentle vertical float animation to earthGroup (±0.6 u, 28 s period)
- Add faint tether beam from Earth down to Nexus platform centre

Fixes #236

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.js

@@ -1446,6 +1446,23 @@ earthGroup.traverse(obj => {
   if (obj.isMesh || obj.isLine) obj.userData.zoomLabel = 'Planet Earth';
 });
 
+// Tether beam — faint line connecting Earth to the Nexus platform center
+(function buildEarthTetherBeam() {
+  const pts = [
+    new THREE.Vector3(0, EARTH_Y - EARTH_RADIUS * 1.15, 0),
+    new THREE.Vector3(0, 0.5, 0),
+  ];
+  const beamGeo = new THREE.BufferGeometry().setFromPoints(pts);
+  const beamMat = new THREE.LineBasicMaterial({
+    color: NEXUS.colors.accent,
+    transparent: true,
+    opacity: 0.08,
+    depthWrite: false,
+    blending: THREE.AdditiveBlending,
+  });
+  scene.add(new THREE.Line(beamGeo, beamMat));
+})();
+
 // === WARP TUNNEL EFFECT ===
 const WarpShader = {
   uniforms: {
@@ -1837,10 +1854,11 @@ function animate() {
     rune.sprite.material.opacity = 0.65 + Math.sin(elapsed * 1.2 + rune.floatPhase) * 0.2;
   }
 
-  // Animate holographic Earth — slow axial rotation, glow pulse
+  // Animate holographic Earth — slow axial rotation, gentle float, glow pulse
   earthMesh.rotation.y = elapsed * EARTH_ROTATION_SPEED;
   earthSurfaceMat.uniforms.uTime.value = elapsed;
   earthGlowLight.intensity = 0.30 + Math.sin(elapsed * 0.7) * 0.12;
+  earthGroup.position.y = EARTH_Y + Math.sin(elapsed * 0.22) * 0.6;
 
   // === WEATHER PARTICLE ANIMATION ===
   if (rainParticles.visible) {