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/key_mapping_tests.rs

@@ -1,5 +1,5 @@
-use std::collections::BTreeMap;
 use domain::{KeyMapping, Value};
+use std::collections::BTreeMap;
 
 #[test]
 fn extracts_value_at_path_and_renames_key() {
@@ -8,11 +8,10 @@ fn extracts_value_at_path_and_renames_key() {
         target_key: "temperature".into(),
     };
 
-    let raw = Value::Object(BTreeMap::from([
-        ("main".into(), Value::Object(BTreeMap::from([
-            ("temp".into(), Value::Number(5.4)),
-        ]))),
-    ]));
+    let raw = Value::Object(BTreeMap::from([(
+        "main".into(),
+        Value::Object(BTreeMap::from([("temp".into(), Value::Number(5.4))])),
+    )]));
 
     let result = mapping.extract(&raw);
     assert_eq!(result, Some(("temperature".into(), Value::Number(5.4))));
@@ -25,9 +24,7 @@ fn returns_none_when_path_does_not_match() {
         target_key: "value".into(),
     };
 
-    let raw = Value::Object(BTreeMap::from([
-        ("other".into(), Value::Number(1.0)),
-    ]));
+    let raw = Value::Object(BTreeMap::from([("other".into(), Value::Number(1.0))]));
 
     assert_eq!(mapping.extract(&raw), None);
 }