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/tests/layout_tests.rs

@@ -1,8 +1,8 @@
-use std::collections::BTreeSet;
 use domain::{
     ContainerNode, Direction, Layout, LayoutChild, LayoutNode, LayoutValidationError, Sizing,
     WidgetId,
 };
+use std::collections::BTreeSet;
 
 fn leaf(id: WidgetId) -> LayoutChild {
     LayoutChild {
@@ -22,14 +22,18 @@ fn row(children: Vec<LayoutChild>) -> LayoutNode {
 
 #[test]
 fn validate_returns_empty_when_all_widgets_exist() {
-    let layout = Layout { root: row(vec![leaf(1), leaf(2)]) };
+    let layout = Layout {
+        root: row(vec![leaf(1), leaf(2)]),
+    };
     let known = BTreeSet::from([1, 2]);
     assert!(layout.validate(&known).is_empty());
 }
 
 #[test]
 fn validate_reports_unknown_widget_ids() {
-    let layout = Layout { root: row(vec![leaf(1), leaf(99)]) };
+    let layout = Layout {
+        root: row(vec![leaf(1), leaf(99)]),
+    };
     let known = BTreeSet::from([1]);
     let errors = layout.validate(&known);
     assert_eq!(errors, vec![LayoutValidationError::UnknownWidget(99)]);
@@ -49,7 +53,9 @@ fn validate_checks_nested_containers() {
         sizing: Sizing::Flex(1),
         node: row(vec![leaf(1), leaf(42)]),
     };
-    let layout = Layout { root: row(vec![inner, leaf(2)]) };
+    let layout = Layout {
+        root: row(vec![inner, leaf(2)]),
+    };
     let known = BTreeSet::from([1, 2]);
     let errors = layout.validate(&known);
     assert_eq!(errors, vec![LayoutValidationError::UnknownWidget(42)]);
@@ -61,6 +67,8 @@ fn widget_ids_collects_all_leaf_ids() {
         sizing: Sizing::Flex(1),
         node: row(vec![leaf(3)]),
     };
-    let layout = Layout { root: row(vec![leaf(1), inner, leaf(2)]) };
+    let layout = Layout {
+        root: row(vec![leaf(1), inner, leaf(2)]),
+    };
     assert_eq!(layout.widget_ids(), BTreeSet::from([1, 2, 3]));
 }