Add expense editing with inline form and PATCH API route

- Add updateExpense() to lib/expenses.ts
- Add PATCH /expenses/:id route with validation and P2025 not-found handling
- Edit button on each expense card pre-fills form; cancel restores add mode
- Submit dynamically PATCHes or POSTs depending on edit state

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/expenses/[id]/route.ts

@@ -1,12 +1,44 @@
 import { Prisma } from "@prisma/client";
 import { NextResponse } from "next/server";
 
-import { removeExpense } from "@/lib/expenses";
+import { removeExpense, updateExpense } from "@/lib/expenses";
+import { expenseInputSchema } from "@/lib/validation";
 
 type RouteContext = {
   params: Promise<{ id: string }>;
 };
 
+export async function PATCH(request: Request, context: RouteContext) {
+  const { id } = await context.params;
+  const payload = await request.json();
+  const parsed = expenseInputSchema.safeParse(payload);
+
+  if (!parsed.success) {
+    return NextResponse.json(
+      { error: parsed.error.issues[0]?.message ?? "Invalid expense payload." },
+      { status: 400 },
+    );
+  }
+
+  try {
+    const expense = await updateExpense(id, {
+      title: parsed.data.title,
+      amountCents: parsed.data.amount,
+      date: parsed.data.date,
+      category: parsed.data.category,
+    });
+    return NextResponse.json({ expense });
+  } catch (error) {
+    if (
+      error instanceof Prisma.PrismaClientKnownRequestError &&
+      error.code === "P2025"
+    ) {
+      return NextResponse.json({ error: "Expense not found." }, { status: 404 });
+    }
+    throw error;
+  }
+}
+
 export async function DELETE(_: Request, context: RouteContext) {
   const { id } = await context.params;