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/widget_config.rs

@@ -1,5 +1,5 @@
-use std::collections::BTreeMap;
 use crate::value_objects::{DisplayHint, KeyMapping, Value, WidgetState};
+use std::collections::BTreeMap;
 
 pub type WidgetId = u16;
 pub type DataSourceId = u16;
@@ -58,7 +58,8 @@ impl WidgetConfig {
     fn truncate_value(value: Value, max_bytes: usize) -> Value {
         match value {
             Value::String(s) if s.len() > max_bytes => {
-                let truncated: String = s.char_indices()
+                let truncated: String = s
+                    .char_indices()
                     .take_while(|(i, _)| *i < max_bytes)
                     .map(|(_, c)| c)
                     .collect();