fix: the flow drawer's head is the version its payload came from, and its callback type says so

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/FlowBuilder.svelte

@@ -1176,7 +1176,7 @@
 			versions: await deployedVersionOptions(),
 			onTakeLatest,
 			draftBase: draftBaseVersion,
-			deployedHead: version != null ? String(version) : undefined,
+			deployedHead: deployedVersionShown != null ? String(deployedVersionShown) : undefined,
 			loadVersion: async (id) => {
 				const v = await FlowService.getFlowVersion({
 					workspace: opWorkspace!,

frontend/src/lib/components/flow_builder.ts

@@ -14,9 +14,10 @@ export type FlowBuilderProps = {
 	 *  draft's `draft_path` so the topbar shows the pending name, so it can't be used
 	 *  to resolve what is actually deployed. */
 	userDraftPath?: string
-	/** Moves the draft's base to the deployed head and keeps its content; offered
-	 *  in the diff drawer while the draft is behind. */
-	onTakeLatest?: () => void | Promise<void>
+	/** Moves the draft's base to the head and keeps its content; offered in the diff
+	 *  drawer while the draft is behind, and called with the version the drawer shows
+	 *  as head. */
+	onTakeLatest?: (head?: string) => void | Promise<void>
 	pathStoreInit?: string | undefined
 	newFlow: boolean
 	selectedId: string | undefined