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

src/app/api/transactions/route.ts

@@ -1,6 +1,7 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getCurrentUser } from "@/lib/auth";
 import { getTransactions } from "@/lib/queries";
+import { queryRaw } from "@/lib/db";
 
 export async function GET(req: NextRequest) {
   const user = await getCurrentUser(req);
@@ -23,3 +24,54 @@ export async function GET(req: NextRequest) {
 
   return NextResponse.json(result);
 }
+
+export async function POST(req: NextRequest) {
+  const user = await getCurrentUser(req);
+  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 403 });
+
+  const body = await req.json() as {
+    date: string;
+    description: string;
+    amount: number;
+    transaction_type?: string;
+    merchant_normalized?: string;
+    category?: string;
+    splits?: { participant_id: number; share_percent: number }[];
+  };
+
+  if (!body.date || !body.description || body.amount == null) {
+    return NextResponse.json({ error: "date, description, amount are required" }, { status: 400 });
+  }
+
+  // Insert manual transaction with no statement (statement_id = NULL, owner_id set directly)
+  const txRows = await queryRaw<{ id: number }>(
+    `INSERT INTO transactions (statement_id, owner_id, transaction_date, description, amount, transaction_type, merchant_normalized, category, row_index)
+     VALUES (NULL, $1, $2, $3, $4, $5, $6, $7, (
+       SELECT COALESCE(MAX(row_index), -1) + 1 FROM transactions WHERE owner_id = $1 AND statement_id IS NULL
+     ))
+     RETURNING id`,
+    [
+      user.id,
+      body.date,
+      body.description,
+      body.amount,
+      body.transaction_type || "debit",
+      body.merchant_normalized || null,
+      body.category || null,
+    ]
+  );
+  const transactionId = txRows[0].id;
+
+  // Insert splits if provided
+  if (body.splits?.length) {
+    for (const s of body.splits) {
+      await queryRaw(
+        `INSERT INTO transaction_splits (transaction_id, participant_id, share_percent)
+         VALUES ($1, $2, $3) ON CONFLICT DO NOTHING`,
+        [transactionId, s.participant_id, s.share_percent]
+      );
+    }
+  }
+
+  return NextResponse.json({ id: transactionId }, { status: 201 });
+}