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

prisma/migrations/0009_split_payments/migration.sql

@@ -0,0 +1,13 @@
+CREATE TABLE split_payments (
+  id SERIAL PRIMARY KEY,
+  from_participant_id INTEGER NOT NULL REFERENCES participants(id),
+  to_participant_id   INTEGER NOT NULL REFERENCES participants(id),
+  amount              DECIMAL(10,2) NOT NULL CHECK (amount > 0),
+  payment_date        DATE NOT NULL,
+  notes               TEXT,
+  linked_transaction_id INTEGER REFERENCES transactions(id) ON DELETE SET NULL,
+  created_at          TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_split_payments_from ON split_payments(from_participant_id);
+CREATE INDEX idx_split_payments_to   ON split_payments(to_participant_id);