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.

app/icon.svg

@@ -0,0 +1,12 @@
+<svg width="32" height="32" viewBox="0 0 30 30" fill="none" xmlns="http://www.w3.org/2000/svg">
+  <rect x="1" y="1" width="28" height="28" rx="8" fill="#2b3033" stroke="rgba(94,224,140,0.45)" />
+  <circle cx="15" cy="15" r="8" stroke="#5ee08c" stroke-width="1.5" />
+  <polyline
+    points="8,15 12,15 14,9 17,21 19,15 22,15"
+    fill="none"
+    stroke="#5ee08c"
+    stroke-width="1.6"
+    stroke-linecap="round"
+    stroke-linejoin="round"
+  />
+</svg>