feat(transactions): manual transaction support and multi-owner query infrastructure

- Add POST /api/transactions to create manual transactions (statement_id=NULL, owner_id set directly)
- Queries switch from JOIN to LEFT JOIN statements so manual transactions are visible
- COALESCE(t.owner_id, s.owner_id) throughout for owner resolution
- Add "Manual" bank filter option in getTransactions
- Search extended to include merchant_normalized override
- Split data fetched via lateral subquery on every transaction row
- getParticipantBalances rewritten as UNION for bidirectional net balances
  (credits/refunds negate, split from either side of the relationship)
- getSharedTransactions: remove my_share_percent from SELECT (fixes GROUP BY error),
  WHERE rewritten as two distinct cases (owner with others split vs participant on others' txn)
- getTransactions: OR EXISTS condition so split participants see shared transactions
- add-transaction-modal component for creating manual transactions with splits
- 0008_my_share_percent migration adds my_share_percent to transaction_overrides

prisma/migrations/0008_my_share_percent/migration.sql

@@ -0,0 +1 @@
+ALTER TABLE "transaction_overrides" ADD COLUMN "my_share_percent" DECIMAL(5,2);

prisma/schema.prisma

@@ -13,6 +13,7 @@ model transaction_overrides {
   merchant_normalized String?
   category_override   String?
   notes               String?
+  my_share_percent    Decimal?  @db.Decimal(5, 2)
   updated_at          DateTime  @default(now()) @updatedAt
 }