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/immer/src/utils/env.ts

@@ -0,0 +1,18 @@
+// Should be no imports here!
+
+/**
+ * The sentinel value returned by producers to replace the draft with undefined.
+ */
+export const NOTHING: unique symbol = Symbol.for("immer-nothing")
+
+/**
+ * To let Immer treat your class instances as plain immutable objects
+ * (albeit with a custom prototype), you must define either an instance property
+ * or a static property on each of your custom classes.
+ *
+ * Otherwise, your class instance will never be drafted, which means it won't be
+ * safe to mutate in a produce callback.
+ */
+export const DRAFTABLE: unique symbol = Symbol.for("immer-draftable")
+
+export const DRAFT_STATE: unique symbol = Symbol.for("immer-state")