feat: apply imported design (bento grid, real elevation tokens, live indicator)

Imports the visual direction from a claude.ai/design session ("PulseNode
Dashboard.dc.html") and reconciles it with the app's existing choices
rather than a wholesale swap: kept the IBM Plex Sans/Mono + Space
Grotesk typography and the pulse-green accent concept from the earlier
pass, adopted the imported design's structural ideas - a real
tonal/elevation token system (derived divider/muted colors via
color-mix instead of hardcoded per-theme duplicates, actual box-shadow
elevation), a 12-column bento grid with per-widget-type column spans
instead of uniform equal-width cards, fading-edge section dividers,
and a shared WidgetCard/MetricBar/StatusTag vocabulary so every widget
type stops repeating its own card markup.

Icons come from @phosphor-icons/react's /ssr entrypoint (bundled at
build time) rather than the imported design's unpkg.com CDN script -
that script is fine for the standalone design-tool preview, but a
runtime third-party call would break the self-hosted-only principle
already established (fonts self-hosted via next/font, no external
requests). New app/icon.svg reuses the same nav badge mark as the
favicon, replacing the never-touched create-next-app default.

Two new pieces of real functionality prompted by the design's mockup
toast/live-indicator, not just decoration: a "live" WebSocket
connection-status indicator (lib/ws/client.ts now tracks and exposes
real connection state), and a toast that fires on actual config:update
and config:error events - the latter finally surfaces config validation
failures in the browser, previously visible only in server logs despite
being designed for exactly this back in M1.

components/layout/GroupSection.tsx

@@ -4,11 +4,14 @@ import { widgetRegistry } from "@/components/widgets/registry";
 
 export function GroupSection({ group, groupIndex }: { group: Group; groupIndex: number }) {
   return (
-    <section className="flex flex-col gap-3">
-      <h2 className="text-xs font-semibold tracking-wide text-fg-muted uppercase">
-        {group.name}
-      </h2>
-      <div className="grid grid-cols-[repeat(auto-fill,minmax(220px,1fr))] gap-3">
+    <section className="mt-8 flex flex-col gap-4">
+      <div className="flex items-baseline gap-2.5">
+        <h6 className="m-0 text-xs font-semibold tracking-wide text-fg-muted uppercase">
+          {group.name}
+        </h6>
+        <div className="pn-hr" />
+      </div>
+      <div className="pn-bento gap-4">
         {group.widgets.map((widget, index) => {
           const Component = widgetRegistry[widget.type];
           const id = widgetId(groupIndex, index);