per-source polling, initial client state, webhook, preview, client tracking

- per-source poll intervals: spawn task per source with own interval,
  manager re-checks sources every 30s for add/remove
- initial screen update on TCP connect: send layout + widget states
- client tracking: ClientRegistry port, GET /api/clients, dashboard list
- webhook adapter: POST /api/webhook/{source_id} feeds data into projection
- widget preview: GET /api/widgets/{id}/preview returns current state
- serve SPA from Axum: ServeDir + index.html fallback via KFRAME_SPA_DIR
- layout builder delete confirmation with AlertDialog
- form validation: required fields disable save button
- guide page at /guide
- fix architecture: ClientDto to api-types, ClientRegistry + WidgetStateReader
  ports in domain, DataProjection has internal Mutex, no adapter cross-deps
- ESP32: full screen clear on layout change (stale pixel fix)

spa/src/components/app-shell.tsx

@@ -18,6 +18,7 @@ import {
   Box,
   Layers,
   Save,
+  BookOpen,
 } from "lucide-react"
 
 const NAV = [
@@ -26,6 +27,7 @@ const NAV = [
   { to: "/widgets", label: "Widgets", icon: Box },
   { to: "/layout", label: "Layout", icon: Layers },
   { to: "/presets", label: "Presets", icon: Save },
+  { to: "/guide", label: "Guide", icon: BookOpen },
 ] as const
 
 export function AppShell({ children }: { children: React.ReactNode }) {