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)

spa/.prettierrc

@@ -0,0 +1,11 @@
+{
+  "endOfLine": "lf",
+  "semi": false,
+  "singleQuote": false,
+  "tabWidth": 2,
+  "trailingComma": "es5",
+  "printWidth": 80,
+  "plugins": ["prettier-plugin-tailwindcss"],
+  "tailwindStylesheet": "src/index.css",
+  "tailwindFunctions": ["cn", "cva"]
+}