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-array/src/superset.js

@@ -0,0 +1,19 @@
+export default function superset(values, other) {
+  const iterator = values[Symbol.iterator](), set = new Set();
+  for (const o of other) {
+    const io = intern(o);
+    if (set.has(io)) continue;
+    let value, done;
+    while (({value, done} = iterator.next())) {
+      if (done) return false;
+      const ivalue = intern(value);
+      set.add(ivalue);
+      if (Object.is(io, ivalue)) break;
+    }
+  }
+  return true;
+}
+
+function intern(value) {
+  return value !== null && typeof value === "object" ? value.valueOf() : value;
+}