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/adapters/http-api/src/routes/clients.rs

@@ -0,0 +1,25 @@
+use crate::AppState;
+use api_types::ClientDto;
+use axum::extract::State;
+use axum::response::Json;
+use domain::{ClientRegistry, ConfigRepository, EventPublisher};
+
+type S<C, E, W, B, R> = State<AppState<C, E, W, B, R>>;
+
+pub async fn list_clients<C, E, W, B, R>(State(state): S<C, E, W, B, R>) -> Json<Vec<ClientDto>>
+where
+    C: ConfigRepository,
+    C::Error: std::fmt::Debug,
+    E: EventPublisher,
+    E::Error: std::fmt::Debug,
+    R: ClientRegistry,
+{
+    Json(
+        state
+            .clients
+            .list_clients()
+            .iter()
+            .map(ClientDto::from)
+            .collect(),
+    )
+}