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)

Cargo.toml

@@ -39,7 +39,7 @@ http-api = { path = "crates/adapters/http-api" }
 media-adapter = { path = "crates/adapters/media" }
 rss-adapter = { path = "crates/adapters/rss" }
 axum = { version = "0.8", features = ["macros"] }
-tower-http = { version = "0.6", features = ["cors"] }
+tower-http = { version = "0.6", features = ["cors", "fs"] }
 api-types = { path = "crates/api-types" }
 thiserror = "2.0"
 anyhow = "1.0"