Add settings page for managing forecasting API key and URL via UI

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

frontend/node_modules/d3-shape/src/path.js

@@ -0,0 +1,19 @@
+import {Path} from "d3-path";
+
+export function withPath(shape) {
+  let digits = 3;
+
+  shape.digits = function(_) {
+    if (!arguments.length) return digits;
+    if (_ == null) {
+      digits = null;
+    } else {
+      const d = Math.floor(_);
+      if (!(d >= 0)) throw new RangeError(`invalid digits: ${_}`);
+      digits = d;
+    }
+    return shape;
+  };
+
+  return () => new Path(digits);
+}