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/domain/src/lib.rs

@@ -10,7 +10,10 @@ pub use entities::{
     LayoutPreset, LayoutPresetId, WidgetConfig, WidgetId,
 };
 pub use events::DomainEvent;
-pub use ports::{BroadcastPort, ConfigRepository, DataSourcePort, EventPublisher};
+pub use ports::{
+    BroadcastPort, ClientRegistry, ConfigRepository, ConnectedClient, DataSourcePort,
+    EventPublisher, WidgetStateReader,
+};
 pub use value_objects::{
     ContainerNode, Direction, DisplayHint, KeyMapping, Layout, LayoutChild, LayoutNode,
     LayoutValidationError, Sizing, Value, WidgetError, WidgetState,