Address #124 review: a failed history refetch must not undo anything

Gadfly (error-handling lens): react-query keeps the stale pages in `data`
when a refetch fails, so `useUndoLast` would fall through and revert the step
BEFORE the one just made — the exact outcome the refetch exists to prevent.
Bail out with a toast unless the refetch succeeded.

Co-Authored-By: Claude Fable 5 <[email protected]>

web/src/editor/useUndoLast.ts

@@ -1,4 +1,5 @@
 import { useCallback } from 'react'
+import { toast } from '@/components/ui/toast'
 import { totalChanges, useGardenHistory, useUndo, type ChangeSet } from '@/lib/history'
 
 /** The newest change set still in effect — not already reverted, and not itself
@@ -23,10 +24,16 @@ export function useUndoLast(gardenId: number, enabled: boolean) {
 
   const undoLast = useCallback(async () => {
     const fresh = await history.refetch()
-    const list = fresh.data?.pages.flatMap((p) => p.changeSets) ?? sets
-    const t = latestUndoable(list)
+    // A failed refetch keeps the STALE pages in `data` (react-query doesn't
+    // clear them), so falling through here would pick the step before the one
+    // just made. Undoing nothing is the only safe answer until it can be read.
+    if (fresh.status !== 'success') {
+      toast.error("Couldn't re-read the history, so nothing was undone — try again.")
+      return
+    }
+    const t = latestUndoable(fresh.data.pages.flatMap((p) => p.changeSets))
     if (t) undo.undo(t)
-  }, [history, undo, sets])
+  }, [history, undo])
 
   return {
     history,