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/tcp-server/src/client_tracker.rs

@@ -0,0 +1,48 @@
+use domain::{ClientRegistry, ConnectedClient};
+use std::net::SocketAddr;
+use std::sync::Mutex;
+use std::time::SystemTime;
+
+#[derive(Default)]
+pub struct ClientTracker {
+    clients: Mutex<Vec<ConnectedClient>>,
+}
+
+impl ClientTracker {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn add(&self, addr: SocketAddr) {
+        let info = ConnectedClient {
+            addr: addr.to_string(),
+            connected_at: SystemTime::now()
+                .duration_since(SystemTime::UNIX_EPOCH)
+                .unwrap_or_default()
+                .as_secs(),
+        };
+        self.clients.lock().unwrap().push(info);
+    }
+
+    pub fn remove(&self, addr: SocketAddr) {
+        let addr_str = addr.to_string();
+        self.clients.lock().unwrap().retain(|c| c.addr != addr_str);
+    }
+}
+
+impl ClientRegistry for ClientTracker {
+    fn add_client(&self, addr: &str, connected_at: u64) {
+        self.clients.lock().unwrap().push(ConnectedClient {
+            addr: addr.to_string(),
+            connected_at,
+        });
+    }
+
+    fn remove_client(&self, addr: &str) {
+        self.clients.lock().unwrap().retain(|c| c.addr != addr);
+    }
+
+    fn list_clients(&self) -> Vec<ConnectedClient> {
+        self.clients.lock().unwrap().clone()
+    }
+}