Show earth curvature in contact elevation profile

Terrain elevations now include the earth bulge correction, so the
profile visually humps up in the middle relative to the straight
LOS beam — showing how the curved earth rises into the signal path.
Uses actual k-factor from HRRR refractivity when available, otherwise
standard 4/3. For a 300 km path at k=4/3, the midpoint bulge is
~2,650 m (8,700 ft).

assets/js/elevation_profile_hook.js

@@ -67,10 +67,20 @@ export const ElevationProfile = {
   renderChart(data) {
     const points = data.points
     const distances = points.map(p => p.dist_km * KM2MI)
-    const elevations = points.map(p => p.elev * M2FT)
+    const showFresnel = data.freq_mhz >= 1000
+
+    // Earth curvature: add bulge to terrain so it curves up in the middle
+    // relative to the straight LOS beam. bulge(f) = d1*d2 / (2*k*R)
+    const k = data.k_factor || (4 / 3)
+    const R = 6371000 // earth radius in meters
+    const dTotal = (data.dist_km || 0) * 1000
+    const elevations = points.map((p, i) => {
+      const f = points.length > 1 ? i / (points.length - 1) : 0
+      const bulge = (f * dTotal * (1 - f) * dTotal) / (2 * k * R)
+      return (p.elev + bulge) * M2FT
+    })
     const losLine = points.map(p => p.beam * M2FT)
     const fresnelLower = points.map(p => (p.beam - p.r1) * M2FT)
-    const showFresnel = data.freq_mhz >= 1000
 
     const ducts = (data.ducts || []).map(d => ({
       base_ft: d.base_m_msl * M2FT,

lib/microwaveprop/terrain/terrain_analysis.ex

@@ -89,6 +89,7 @@ defmodule Microwaveprop.Terrain.TerrainAnalysis do
       points: points,
       diffraction_db: diffraction_db,
       verdict: verdict,
+      k_factor: k,
       max_elevation_m: max_elevation_m,
       min_clearance_m: min_clearance_m,
       obstructed_count: length(obstructed),

lib/microwaveprop_web/live/contact_live/show.ex

@@ -1103,6 +1103,7 @@ defmodule MicrowavepropWeb.ContactLive.Show do
           end),
         freq_mhz: freq_ghz * 1000,
         dist_km: dist_km,
+        k_factor: analysis.k_factor,
         fwd_az: fwd_az,
         rev_az: rev_az,
         fwd_el: fwd_el,