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/entities/mod.rs

@@ -1,7 +1,9 @@
-mod widget_config;
 mod data_source;
 mod layout_preset;
+mod widget_config;
 
-pub use widget_config::{WidgetConfig, WidgetId};
-pub use data_source::{DataSource, DataSourceId, DataSourceType, DataSourceConfig, DataSourceValidationError};
+pub use data_source::{
+    DataSource, DataSourceConfig, DataSourceId, DataSourceType, DataSourceValidationError,
+};
 pub use layout_preset::{LayoutPreset, LayoutPresetId};
+pub use widget_config::{WidgetConfig, WidgetId};