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)

crates/client-esp32/src/tasks/render.rs

@@ -21,9 +21,10 @@ pub fn run(
     loop {
         match rx.recv_timeout(RENDER_POLL_INTERVAL) {
             Ok(msg) => {
+                let is_screen_update = matches!(msg, ServerMessage::ScreenUpdate { .. });
                 let repaints = app.handle_message(msg);
 
-                if !repaints.is_empty() && first_update {
+                if !repaints.is_empty() && (first_update || is_screen_update) {
                     display.fill_background(SCREEN).unwrap();
                     first_update = false;
                 }