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/api/rules/apply/route.ts

@@ -9,10 +9,16 @@ interface Condition {
   value: string;
 }
 
+interface SplitEntry {
+  participant_id: number;
+  share_percent: number;
+}
+
 interface Actions {
   set_category?: string;
   add_tag_ids?: number[];
   set_merchant?: string;
+  apply_split?: SplitEntry[];
 }
 
 interface TxFields {
@@ -108,6 +114,18 @@ export async function POST(req: NextRequest) {
           );
         }
       }
+
+      if (actions.apply_split?.length) {
+        // Delete existing splits then insert new ones
+        await queryRaw(`DELETE FROM transaction_splits WHERE transaction_id = $1`, [tx.id]);
+        for (const s of actions.apply_split) {
+          await queryRaw(
+            `INSERT INTO transaction_splits (transaction_id, participant_id, share_percent)
+             VALUES ($1, $2, $3) ON CONFLICT DO NOTHING`,
+            [tx.id, s.participant_id, s.share_percent]
+          );
+        }
+      }
     }
   }
 

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()); }}
         />
       )}

src/components/split-modal.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useState, useEffect } from "react";
-import { useParticipants, useSetSplits, useTransactionSplits, useBulkAction } from "@/lib/hooks";
+import { useParticipants, useSetSplits, useTransactionSplits, useBulkAction, useCreateRule } from "@/lib/hooks";
 
 interface Split {
   participant_id: number;
@@ -13,10 +13,11 @@ interface Props {
   transactionIds?: number[];
   amount?: number;
   description: string;
+  merchant?: string;
   onClose: () => void;
 }
 
-export function SplitModal({ transactionId, transactionIds, amount, description, onClose }: Props) {
+export function SplitModal({ transactionId, transactionIds, amount, description, merchant, onClose }: Props) {
   const isBulk = !!transactionIds && transactionIds.length > 0;
   const singleId = transactionId ?? 0;
 
@@ -24,9 +25,12 @@ export function SplitModal({ transactionId, transactionIds, amount, description,
   const { data: existingSplits } = useTransactionSplits(isBulk ? 0 : singleId);
   const setSplits = useSetSplits();
   const bulkAction = useBulkAction();
+  const createRule = useCreateRule();
 
   const [splits, setSplitsState] = useState<Split[]>([]);
   const [error, setError] = useState("");
+  const [saveAsRule, setSaveAsRule] = useState(false);
+  const [ruleSaved, setRuleSaved] = useState(false);
 
   // Initialise: bulk always defaults to 100% Me; single loads existing splits
   useEffect(() => {
@@ -94,7 +98,20 @@ export function SplitModal({ transactionId, transactionIds, amount, description,
       } else {
         await setSplits.mutateAsync({ transactionId: singleId, splits });
       }
+      if (saveAsRule && !isBulk) {
+        const matchValue = merchant || (description.split(" ")[0] ?? description);
+        await createRule.mutateAsync({
+          name: `Split: ${merchant || description}`,
+          conditions: [{ field: "merchant_normalized", operator: "contains", value: matchValue }],
+          actions: { apply_split: splits },
+          enabled: true,
+          priority: 0,
+        });
+        setRuleSaved(true);
+        setTimeout(onClose, 1200);
+      } else {
         onClose();
+      }
     } catch (e) {
       setError(e instanceof Error ? e.message : "Failed to save splits");
     }
@@ -174,6 +191,22 @@ export function SplitModal({ transactionId, transactionIds, amount, description,
 
         {error && <p className="text-red-400 text-sm mb-3">{error}</p>}
 
+        {ruleSaved && (
+          <p className="text-green-400 text-sm mb-3">Rule saved — future matching transactions will be split the same way.</p>
+        )}
+
+        {!isBulk && (
+          <label className="flex items-center gap-2 text-sm text-zinc-400 mb-3 cursor-pointer select-none">
+            <input
+              type="checkbox"
+              checked={saveAsRule}
+              onChange={(e) => setSaveAsRule(e.target.checked)}
+              className="accent-blue-500"
+            />
+            Also save as rule for <span className="text-zinc-200 font-medium">{merchant || description.split(" ")[0]}</span>
+          </label>
+        )}
+
         <div className="flex gap-3">
           <button
             onClick={onClose}

src/lib/hooks.ts

@@ -360,7 +360,7 @@ export interface RuleRow {
   id: number;
   name: string;
   conditions: { field: string; operator: string; value: string }[];
-  actions: { set_category?: string; add_tag_ids?: number[]; set_merchant?: string };
+  actions: { set_category?: string; add_tag_ids?: number[]; set_merchant?: string; apply_split?: { participant_id: number; share_percent: number }[] };
   enabled: boolean;
   priority: number;
   created_at: string;