feat: add dereth.png map background to radar canvas

Draws a rotated/scaled slice of the overworld map behind radar dots.
Map clips to the radar circle, rotates with heading-up mode, and
renders at 40% opacity so dots remain visible on top.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/script.js

@@ -3657,6 +3657,10 @@ function showRadarWindow(name) {
   listContainer.appendChild(listBody);
   content.appendChild(listContainer);
 
+  // Load map image for radar background
+  const radarMapImg = new Image();
+  radarMapImg.src = 'dereth.png';
+
   // Store refs on the window
   win._radarCanvas = canvas;
   win._radarListBody = listBody;
@@ -3664,6 +3668,7 @@ function showRadarWindow(name) {
   win._radarRangeDisplay = rangeDisplay;
   win._radarSelectedId = null;
   win._radarLastObjects = [];  // cache for click hit-testing
+  win._radarMapImg = radarMapImg;
 
   // Scroll-wheel zoom on canvas
   canvas.addEventListener('wheel', (e) => {
@@ -3726,6 +3731,39 @@ function updateRadarWindow(msg) {
   ctx.arc(cx, cy, cx, 0, Math.PI * 2);
   ctx.fill();
 
+  // Map background (clip to circle, rotate to heading-up)
+  const mapImg = win._radarMapImg;
+  if (mapImg && mapImg.complete && mapImg.naturalWidth) {
+    ctx.save();
+    // Clip to radar circle
+    ctx.beginPath();
+    ctx.arc(cx, cy, cx - 1, 0, Math.PI * 2);
+    ctx.clip();
+    // Map coordinate system: -102.1 to 102.1 for both axes
+    const mapCoordRange = 204.2;
+    const pixPerCoord = mapImg.naturalWidth / mapCoordRange;
+    // Player position in map pixel space
+    const mapPx = (playerEW + 102.1) * pixPerCoord;
+    const mapPy = (102.1 - playerNS) * pixPerCoord; // Y flipped (north = top)
+    // How many map pixels correspond to the radar range
+    const mapPixelsPerRadarPixel = (range * pixPerCoord) / (size / 2);
+    // Draw rotated map centered on player
+    ctx.translate(cx, cy);
+    ctx.rotate(-playerHeading * Math.PI / 180); // heading-up rotation
+    ctx.globalAlpha = 0.4;
+    const srcSize = range * pixPerCoord * 2;
+    ctx.drawImage(mapImg,
+      mapPx - srcSize / 2, mapPy - srcSize / 2, srcSize, srcSize,
+      -cx, -cy, size, size
+    );
+    ctx.globalAlpha = 1.0;
+    ctx.restore();
+    // Re-clip for remaining draws
+    ctx.beginPath();
+    ctx.arc(cx, cy, cx, 0, Math.PI * 2);
+    ctx.clip();
+  }
+
   // Range rings
   ctx.strokeStyle = '#333';
   ctx.lineWidth = 1;