Keep source URLs in plans and expose an editable source field

public/index.html

@@ -408,6 +408,10 @@
                     name="0.1"
                     placeholder="e.g. Finca El Ocaso"
                 /></label>
+                <label class="field"><span class="field-label">Source URL</span>
+                  <input class="field-input" type="url" name="sourceUrl" placeholder="https://example.com/coffee" />
+                  <span class="field-note">Coffee product page or other source for this plan.</span>
+                </label>
                 <label class="field" data-fid="0.2"
                   ><span class="field-label">Date</span
                   ><input class="field-input" type="date" name="0.2"

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] ?? "") !== "")