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.

lib/config/schema.ts

@@ -0,0 +1,51 @@
+import { z } from "zod";
+
+const searchEngineSchema = z.enum(["duckduckgo", "google", "bing"]);
+
+export const bookmarkWidgetSchema = z.object({
+  type: z.literal("bookmark"),
+  name: z.string(),
+  href: z.string().url(),
+  description: z.string().optional(),
+});
+
+export const searchWidgetSchema = z.object({
+  type: z.literal("search"),
+  engines: z.array(searchEngineSchema).min(1).default(["duckduckgo"]),
+  defaultEngine: searchEngineSchema.optional(),
+});
+
+export const widgetSchema = z.discriminatedUnion("type", [
+  bookmarkWidgetSchema,
+  searchWidgetSchema,
+]);
+
+export const groupSchema = z.object({
+  name: z.string(),
+  widgets: z.array(widgetSchema),
+});
+
+export const themeSchema = z
+  .object({
+    mode: z.enum(["dark", "light", "auto"]).default("dark"),
+    variables: z.record(z.string(), z.string()).default({}),
+    customCssPath: z.string().optional(),
+  })
+  .default({ mode: "dark", variables: {} });
+
+export const settingsSchema = z
+  .object({
+    title: z.string().default("PulseNode"),
+  })
+  .default({ title: "PulseNode" });
+
+export const configSchema = z.object({
+  theme: themeSchema,
+  settings: settingsSchema,
+  groups: z.array(groupSchema).default([]),
+});
+
+export type Config = z.infer<typeof configSchema>;
+export type Widget = z.infer<typeof widgetSchema>;
+export type Group = z.infer<typeof groupSchema>;
+export type SearchEngine = z.infer<typeof searchEngineSchema>;