feat(merchants): scatter plot, merchant profiles, and per-merchant transaction history

- /merchants page with spend-vs-frequency scatter chart (click to open profile)
- Merchant profile drawer: stats, monthly trend line, full transaction history
- /api/analytics/merchants: split-adjusted merchant aggregates + monthly trends
- /api/analytics/merchants/[merchant]: per-merchant transaction list
- Add Merchants nav item to sidebar

src/components/sidebar.tsx

@@ -9,6 +9,7 @@ const NAV_ITEMS = [
   { href: "/shared", label: "Shared", icon: "users" },
   { href: "/budget", label: "Analytics", icon: "bar-chart" },
   { href: "/insights", label: "Insights", icon: "lightbulb" },
+  { href: "/merchants", label: "Merchants", icon: "store" },
   { href: "/tags", label: "Tags", icon: "tag" },
   { href: "/rules", label: "Rules", icon: "settings" },
 ];
@@ -50,6 +51,11 @@ const ICONS: Record<string, React.ReactNode> = {
       <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M9.663 17h4.673M12 3v1m6.364 1.636l-.707.707M21 12h-1M4 12H3m1.343-5.657l-.707-.707m2.828 9.9a5 5 0 117.072 0l-.548.547A3.374 3.374 0 0014 18.469V19a2 2 0 11-4 0v-.531c0-.895-.356-1.754-.988-2.386l-.548-.547z" />
     </svg>
   ),
+  store: (
+    <svg className="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+      <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M3 3h2l.4 2M7 13h10l4-8H5.4M7 13L5.4 5M7 13l-2.293 2.293c-.63.63-.184 1.707.707 1.707H17m0 0a2 2 0 100 4 2 2 0 000-4zm-8 2a2 2 0 11-4 0 2 2 0 014 0z" />
+    </svg>
+  ),
 };
 
 export function Sidebar() {