feat(shared): replace settle buttons with payment ledger

- New split_payments table records actual payments between participants
- Balance = total split obligations - total payments (splits never marked settled)
- Record Payment modal per participant: direction toggle, amount pre-filled with balance, date, notes
- Payment history inline on each balance card with +/- display and delete
- Per-transaction Settle button removed; Action column removed from shared table
- Splits always show the true cost breakdown regardless of payment state

src/lib/hooks.ts

@@ -257,11 +257,43 @@ export function useSetSplits() {
   });
 }
 
-export function useSettleSplits() {
+export interface SplitPayment {
+  id: number;
+  from_participant_id: number;
+  from_name: string;
+  to_participant_id: number;
+  to_name: string;
+  amount: number;
+  payment_date: string;
+  notes: string | null;
+  linked_transaction_id: number | null;
+  created_at: string;
+}
+
+export function usePaymentHistory(participantId: number | null) {
+  return useQuery<SplitPayment[]>({
+    queryKey: ["split-payments", participantId],
+    queryFn: async () => {
+      if (!participantId) return [];
+      const res = await fetch(`/api/split-payments?participant_id=${participantId}`);
+      return res.json();
+    },
+    enabled: !!participantId,
+  });
+}
+
+export function useRecordPayment() {
   const qc = useQueryClient();
   return useMutation({
-    mutationFn: async (body: { participant_id?: number; split_ids?: number[] }) => {
-      const res = await fetch("/api/splits/settle", {
+    mutationFn: async (body: {
+      from_participant_id: number;
+      to_participant_id: number;
+      amount: number;
+      payment_date: string;
+      notes?: string;
+      linked_transaction_id?: number;
+    }) => {
+      const res = await fetch("/api/split-payments", {
         method: "POST",
         headers: { "Content-Type": "application/json" },
         body: JSON.stringify(body),
@@ -269,8 +301,22 @@ export function useSettleSplits() {
       return res.json();
     },
     onSuccess: () => {
-      qc.invalidateQueries({ queryKey: ["shared-transactions"] });
       qc.invalidateQueries({ queryKey: ["participant-balances"] });
+      qc.invalidateQueries({ queryKey: ["split-payments"] });
+    },
+  });
+}
+
+export function useDeletePayment() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: async (id: number) => {
+      const res = await fetch(`/api/split-payments?id=${id}`, { method: "DELETE" });
+      return res.json();
+    },
+    onSuccess: () => {
+      qc.invalidateQueries({ queryKey: ["participant-balances"] });
+      qc.invalidateQueries({ queryKey: ["split-payments"] });
     },
   });
 }