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/README.md

@@ -0,0 +1,21 @@
+# React + TypeScript + Vite + shadcn/ui
+
+This is a template for a new Vite project with React, TypeScript, and shadcn/ui.
+
+## Adding components
+
+To add components to your app, run the following command:
+
+```bash
+npx shadcn@latest add button
+```
+
+This will place the ui components in the `src/components` directory.
+
+## Using components
+
+To use the components in your app, import them as follows:
+
+```tsx
+import { Button } from "@/components/ui/button"
+```