feat(splits): save split as rule from split modal

- Checkbox in split modal: 'Also save as rule for <merchant>'
- Creates a rule with apply_split action storing the participant shares
- Rules engine now handles apply_split: deletes existing splits and re-applies
- Bulk split mode hides the checkbox (rule wouldn't make sense for ad-hoc bulk)

src/app/transactions/page.tsx

@@ -232,7 +232,7 @@ function TransactionsContent() {
   const [selected, setSelected] = useState<Set<number>>(new Set());
   const [bulkCategory, setBulkCategory] = useState("");
   const [bulkTagId, setBulkTagId] = useState("");
-  const [splitModal, setSplitModal] = useState<{ transactionId?: number; transactionIds?: number[]; amount?: number; description: string } | null>(null);
+  const [splitModal, setSplitModal] = useState<{ transactionId?: number; transactionIds?: number[]; amount?: number; description: string; merchant?: string } | null>(null);
   const [rulePrompt, setRulePrompt] = useState<{
     tx: { id: number; effective_merchant: string; description: string; bank_name: string };
     field: "category" | "merchant";
@@ -543,7 +543,7 @@ function TransactionsContent() {
                   </td>
                   <td className="p-2">
                     <button
-                      onClick={() => setSplitModal({ transactionId: t.id, amount: t.amount, description: t.description, transactionIds: undefined })}
+                      onClick={() => setSplitModal({ transactionId: t.id, amount: t.amount, description: t.description, merchant: t.effective_merchant || undefined, transactionIds: undefined })}
                       className="text-xs text-zinc-500 hover:text-zinc-200 px-2 py-0.5 rounded hover:bg-zinc-800 transition-colors"
                       title="Split this transaction"
                     >
@@ -564,6 +564,7 @@ function TransactionsContent() {
           transactionIds={splitModal.transactionIds}
           amount={splitModal.amount}
           description={splitModal.description}
+          merchant={splitModal.merchant}
           onClose={() => { setSplitModal(null); if (splitModal.transactionIds) setSelected(new Set()); }}
         />
       )}