feat(transactions): save-as-rule prompt after merchant/category edit

After changing a merchant name or category inline, a toast-style prompt
appears offering to save it as a rule. Shows a preview of the condition
and action before saving. Dismissable without creating a rule.

src/app/transactions/page.tsx

@@ -2,7 +2,7 @@
 
 import { useState, useCallback, Suspense } from "react";
 import { useSearchParams } from "next/navigation";
-import { useTransactions, useBanks, useUpdateTransaction, useBulkAction, useTags, useStatement } from "@/lib/hooks";
+import { useTransactions, useBanks, useUpdateTransaction, useBulkAction, useTags, useStatement, useCreateRule } from "@/lib/hooks";
 import { CATEGORIES, formatCategory } from "@/lib/categories";
 import { SplitModal } from "@/components/split-modal";
 import { TagPicker } from "@/components/tag-picker";
@@ -71,6 +71,80 @@ function EditableTypeBadge({ type, onSave }: { type: string; onSave: (t: string)
   );
 }
 
+// Prompt shown after a merchant/category edit — offers to save it as a rule
+function SaveAsRulePrompt({
+  tx,
+  field,
+  newValue,
+  onDone,
+}: {
+  tx: { id: number; effective_merchant: string; description: string; bank_name: string };
+  field: "category" | "merchant";
+  newValue: string;
+  onDone: () => void;
+}) {
+  const createRule = useCreateRule();
+  const [saving, setSaving] = useState(false);
+
+  // Build a sensible default rule from the transaction context
+  const merchantMatch = tx.effective_merchant || tx.description;
+  const defaultName =
+    field === "category"
+      ? `${merchantMatch} → ${formatCategory(newValue)}`
+      : `Rename ${tx.effective_merchant || tx.description} → ${newValue}`;
+
+  const conditions = [
+    {
+      field: "description",
+      operator: "contains",
+      value: (tx.effective_merchant || tx.description).split(" ")[0] ?? "",
+    },
+  ];
+  const actions =
+    field === "category"
+      ? { set_category: newValue }
+      : { set_merchant: newValue };
+
+  async function save() {
+    setSaving(true);
+    await createRule.mutateAsync({ name: defaultName, conditions, actions, enabled: true, priority: 0 });
+    onDone();
+  }
+
+  return (
+    <div className="fixed bottom-4 right-4 z-50 bg-zinc-800 border border-zinc-600 rounded-xl shadow-2xl p-4 w-80 text-sm">
+      <p className="text-zinc-200 font-medium mb-1">Save as rule?</p>
+      <p className="text-zinc-400 text-xs mb-3">
+        Automatically apply this {field === "category" ? "category" : "merchant name"} to future matching transactions.
+      </p>
+      <div className="bg-zinc-900 rounded-lg px-3 py-2 text-xs text-zinc-300 mb-3 space-y-1">
+        <p><span className="text-zinc-500">If</span> description contains <span className="text-white">"{conditions[0].value}"</span></p>
+        <p>
+          <span className="text-zinc-500">Then</span>{" "}
+          {field === "category"
+            ? <>set category → <span className="text-white">{formatCategory(newValue)}</span></>
+            : <>set merchant → <span className="text-white">{newValue}</span></>}
+        </p>
+      </div>
+      <div className="flex gap-2">
+        <button
+          onClick={save}
+          disabled={saving}
+          className="flex-1 bg-blue-600 hover:bg-blue-500 disabled:opacity-50 text-white rounded-lg py-1.5 font-medium transition-colors"
+        >
+          {saving ? "Saving…" : "Save rule"}
+        </button>
+        <button
+          onClick={onDone}
+          className="flex-1 bg-zinc-700 hover:bg-zinc-600 text-zinc-200 rounded-lg py-1.5 transition-colors"
+        >
+          Dismiss
+        </button>
+      </div>
+    </div>
+  );
+}
+
 function InlineEdit({
   value,
   onSave,
@@ -159,6 +233,11 @@ function TransactionsContent() {
   const [bulkCategory, setBulkCategory] = useState("");
   const [bulkTagId, setBulkTagId] = useState("");
   const [splitModal, setSplitModal] = useState<{ transactionId?: number; transactionIds?: number[]; amount?: number; description: string } | null>(null);
+  const [rulePrompt, setRulePrompt] = useState<{
+    tx: { id: number; effective_merchant: string; description: string; bank_name: string };
+    field: "category" | "merchant";
+    newValue: string;
+  } | null>(null);
 
   const { data, isLoading } = useTransactions(filters);
   const { data: banks } = useBanks();
@@ -410,7 +489,10 @@ function TransactionsContent() {
                     <div className="relative">
                       <InlineEdit
                         value={t.effective_merchant || ""}
-                        onSave={(val) => updateTxn.mutate({ id: t.id, merchant_normalized: val })}
+                        onSave={(val) => {
+                          updateTxn.mutate({ id: t.id, merchant_normalized: val });
+                          setRulePrompt({ tx: t, field: "merchant", newValue: val });
+                        }}
                       />
                       {t.merchant_override && (
                         <span className="absolute -left-2 top-1/2 -translate-y-1/2 w-1.5 h-1.5 bg-blue-500 rounded-full" title="Manually overridden" />
@@ -432,7 +514,10 @@ function TransactionsContent() {
                     <div className="relative">
                       <InlineEdit
                         value={t.effective_category}
-                        onSave={(val) => updateTxn.mutate({ id: t.id, category: val })}
+                        onSave={(val) => {
+                          updateTxn.mutate({ id: t.id, category: val });
+                          setRulePrompt({ tx: t, field: "category", newValue: val });
+                        }}
                         type="select"
                         options={categoryOptions}
                       />
@@ -483,6 +568,15 @@ function TransactionsContent() {
         />
       )}
 
+      {rulePrompt && (
+        <SaveAsRulePrompt
+          tx={rulePrompt.tx}
+          field={rulePrompt.field}
+          newValue={rulePrompt.newValue}
+          onDone={() => setRulePrompt(null)}
+        />
+      )}
+
       {/* Pagination */}
       {data && data.total > filters.limit && (
         <div className="flex items-center justify-between mt-4">