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/api-types/src/client.rs

@@ -0,0 +1,17 @@
+use domain::ConnectedClient;
+use serde::Serialize;
+
+#[derive(Serialize)]
+pub struct ClientDto {
+    pub addr: String,
+    pub connected_at: u64,
+}
+
+impl From<&ConnectedClient> for ClientDto {
+    fn from(c: &ConnectedClient) -> Self {
+        Self {
+            addr: c.addr.clone(),
+            connected_at: c.connected_at,
+        }
+    }
+}

crates/api-types/src/lib.rs

@@ -1,8 +1,10 @@
+pub mod client;
 pub mod data_source;
 pub mod layout;
 pub mod preset;
 pub mod widget;
 
+pub use client::ClientDto;
 pub use data_source::DataSourceDto;
 pub use layout::{LayoutChildDto, LayoutDto, LayoutNodeDto, SizingDto};
 pub use preset::{CreatePresetDto, PresetDto};