chore: convert submodules to standard directories for true monorepo structure

vibn-frontend/app/sentry-example-page/page.tsx

@@ -0,0 +1,69 @@
+"use client";
+
+/**
+ * Sentry verification page. Visit /sentry-example-page in any
+ * environment with NEXT_PUBLIC_SENTRY_DSN set, click both buttons,
+ * and confirm two issues land in the Sentry dashboard (one client,
+ * one server). Safe to keep around — it's gated on a click and not
+ * linked from anywhere else in the app.
+ */
+
+import * as Sentry from "@sentry/nextjs";
+
+export default function SentryExamplePage() {
+  return (
+    <main
+      style={{
+        display: "flex",
+        flexDirection: "column",
+        alignItems: "center",
+        justifyContent: "center",
+        minHeight: "100vh",
+        padding: "2rem",
+        fontFamily: "system-ui, -apple-system, sans-serif",
+        gap: "1rem",
+      }}
+    >
+      <h1 style={{ fontSize: "1.25rem" }}>Sentry verification</h1>
+      <p style={{ color: "#666", maxWidth: "32rem", textAlign: "center" }}>
+        Click each button to fire a test error. Both should land in the
+        Sentry dashboard within a few seconds.
+      </p>
+
+      <button
+        onClick={() => {
+          throw new Error("Sentry test (client) — vibn-ai");
+        }}
+        style={btnStyle}
+      >
+        Throw client error
+      </button>
+
+      <button
+        onClick={async () => {
+          await Sentry.startSpan(
+            { name: "sentry-test-server", op: "ui.click" },
+            async () => {
+              const res = await fetch("/api/sentry-example-api");
+              if (!res.ok) {
+                throw new Error("Sentry test (server) — vibn-ai");
+              }
+            },
+          );
+        }}
+        style={btnStyle}
+      >
+        Throw server error
+      </button>
+    </main>
+  );
+}
+
+const btnStyle: React.CSSProperties = {
+  padding: "0.5rem 1rem",
+  borderRadius: "0.375rem",
+  border: "1px solid #ccc",
+  background: "#f5f5f5",
+  cursor: "pointer",
+  fontFamily: "inherit",
+};