feat: add app shell with sidebar navigation, routing, and placeholder pages

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

src/pages/campaigns.tsx

@@ -0,0 +1,12 @@
+import { TopBar } from "@/components/layout/top-bar";
+
+export const CampaignsPage = () => {
+    return (
+        <div className="flex flex-1 flex-col">
+            <TopBar title="Campaigns" />
+            <div className="flex flex-1 items-center justify-center p-8">
+                <p className="text-tertiary">Campaigns — coming soon</p>
+            </div>
+        </div>
+    );
+};