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.

postcss.config.mjs

@@ -0,0 +1,7 @@
+const config = {
+  plugins: {
+    "@tailwindcss/postcss": {},
+  },
+};
+
+export default config;