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/ThemeStyleInjector.tsx

@@ -0,0 +1,8 @@
+export function ThemeStyleInjector({ variables }: { variables: Record<string, string> }) {
+  const entries = Object.entries(variables);
+  if (entries.length === 0) return null;
+
+  const css = `:root{${entries.map(([key, value]) => `${key}:${value};`).join("")}}`;
+
+  return <style id="pulsenode-theme-overrides" dangerouslySetInnerHTML={{ __html: css }} />;
+}