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)

spa/src/pages/data-sources.tsx

@@ -192,7 +192,15 @@ export function DataSourcesPage() {
             <Button variant="outline" onClick={() => setEditing(null)}>
               Cancel
             </Button>
-            <Button onClick={save} disabled={!editing?.name}>
+            <Button
+              onClick={save}
+              disabled={
+                !editing?.name ||
+                (editing.source_type !== "webhook" &&
+                  editing.poll_interval_secs <= 0) ||
+                (editing.source_type !== "webhook" && !editing.url)
+              }
+            >
               Save
             </Button>
           </DialogFooter>