feat: scaffold PulseNode dashboard (M1)

Next.js 16 + TypeScript + Tailwind v4 app with a YAML-driven config
system: schema validation (zod), .env interpolation, and a widget
registry rendering bookmark/search/group widgets. CSS custom
properties (--pn-*) drive theming and are overridable from
config.yml's theme.variables. Config load errors surface as a
readable error page instead of crashing the app.

Hot reload, docker/system/http collectors, and the WebSocket push
layer land in later milestones per the approved plan.

components/layout/Dashboard.tsx

@@ -0,0 +1,15 @@
+import type { Config } from "@/lib/config/schema";
+import { GroupSection } from "./GroupSection";
+
+export function Dashboard({ config }: { config: Config }) {
+  return (
+    <main className="mx-auto flex max-w-6xl flex-col gap-8 px-6 py-10">
+      <header className="flex items-center justify-between">
+        <h1 className="text-lg font-semibold text-fg">{config.settings.title}</h1>
+      </header>
+      {config.groups.map((group) => (
+        <GroupSection key={group.name} group={group} />
+      ))}
+    </main>
+  );
+}