add SPA config UI, wire media/rss adapters, event-driven layout push

- React SPA: dashboard, data sources CRUD, widgets CRUD, layout builder,
  presets. TanStack Router + Query, shadcn/ui, Vite proxy to :3000
- wire media + rss adapters into polling loop, remove xtb source type
- media adapter: read username/password from headers, proper subsonic auth
- event handler: subscribe to LayoutChanged, push screen update to clients
- fix clippy warnings across workspace (Default impls, collapsible ifs,
  redundant closures, is_none_or, unused imports)

crates/domain/src/ports/broadcast.rs

@@ -1,6 +1,6 @@
-use std::future::Future;
 use crate::entities::WidgetId;
 use crate::value_objects::{Layout, WidgetState};
+use std::future::Future;
 
 pub trait BroadcastPort {
     type Error;