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/src/main.tsx

@@ -0,0 +1,26 @@
+import { StrictMode } from "react"
+import { createRoot } from "react-dom/client"
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query"
+import { RouterProvider } from "@tanstack/react-router"
+import { ThemeProvider } from "@/components/theme-provider"
+import { router } from "@/router"
+import "./index.css"
+
+const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      staleTime: 10_000,
+      retry: 1,
+    },
+  },
+})
+
+createRoot(document.getElementById("root")!).render(
+  <StrictMode>
+    <QueryClientProvider client={queryClient}>
+      <ThemeProvider>
+        <RouterProvider router={router} />
+      </ThemeProvider>
+    </QueryClientProvider>
+  </StrictMode>,
+)