Add sidebar navigation with responsive bottom bar

Three pages: AutoTrader, AutoBuyer, AutoRequester. Fixed left sidebar
on desktop (md+), bottom tab bar on mobile.

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

app/autobuyer/page.tsx

@@ -0,0 +1,12 @@
+export default function AutoBuyer() {
+    return (
+        <div className="min-h-screen bg-slate-50 p-8">
+            <div className="max-w-7xl mx-auto">
+                <h1 className="text-2xl font-bold text-slate-900 mb-6">AutoBuyer</h1>
+                <div className="bg-white border border-slate-200 rounded-xl shadow-sm p-8 text-center text-slate-400">
+                    Coming soon
+                </div>
+            </div>
+        </div>
+    );
+}