Implement expense tracking foundation for v1

src/app/income/page.tsx

@@ -0,0 +1,15 @@
+export const metadata = {
+  title: "Income & Paychecks | Monthy Tracker",
+};
+
+export default function IncomePage() {
+  return (
+    <div className="rounded-[2rem] border border-stone-200 bg-white p-8 shadow-[0_24px_60px_rgba(120,90,50,0.08)]">
+      <p className="text-sm font-semibold uppercase tracking-[0.28em] text-stone-500">Coming next</p>
+      <h1 className="mt-3 text-4xl font-semibold text-stone-950">Paycheck tracking lands in the next implementation slice.</h1>
+      <p className="mt-4 max-w-2xl text-lg leading-8 text-stone-600">
+        The data model is already prepared for paychecks. This view will add create, list, and delete flows after expense tracking is validated.
+      </p>
+    </div>
+  );
+}