Keep source URLs in plans and expose an editable source field

public/js/prefill-ui.js

@@ -35,6 +35,7 @@ export function initPrefillPanel({ state, renderFormFromPlan, recompute }) {
 				return;
 			}
 			snapshot = structuredClone(state.plan);
+			state.plan.sourceUrl = url;
 			let applied = 0;
 			for (const [id, value] of Object.entries(body.fields ?? {})) {
 				if (!overwriteBox.checked && (state.plan.fields[id] ?? "") !== "")