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/variance.js

@@ -0,0 +1,25 @@
+export default function variance(values, valueof) {
+  let count = 0;
+  let delta;
+  let mean = 0;
+  let sum = 0;
+  if (valueof === undefined) {
+    for (let value of values) {
+      if (value != null && (value = +value) >= value) {
+        delta = value - mean;
+        mean += delta / ++count;
+        sum += delta * (value - mean);
+      }
+    }
+  } else {
+    let index = -1;
+    for (let value of values) {
+      if ((value = valueof(value, ++index, values)) != null && (value = +value) >= value) {
+        delta = value - mean;
+        mean += delta / ++count;
+        sum += delta * (value - mean);
+      }
+    }
+  }
+  if (count > 1) return sum / (count - 1);
+}