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/router.tsx

@@ -0,0 +1,66 @@
+import {
+  createRootRoute,
+  createRoute,
+  createRouter,
+  Outlet,
+} from "@tanstack/react-router"
+import { AppShell } from "@/components/app-shell"
+import { DashboardPage } from "@/pages/dashboard"
+import { DataSourcesPage } from "@/pages/data-sources"
+import { WidgetsPage } from "@/pages/widgets"
+import { LayoutBuilderPage } from "@/pages/layout-builder"
+import { PresetsPage } from "@/pages/presets"
+
+const rootRoute = createRootRoute({
+  component: () => (
+    <AppShell>
+      <Outlet />
+    </AppShell>
+  ),
+})
+
+const indexRoute = createRoute({
+  getParentRoute: () => rootRoute,
+  path: "/",
+  component: DashboardPage,
+})
+
+const dataSourcesRoute = createRoute({
+  getParentRoute: () => rootRoute,
+  path: "/data-sources",
+  component: DataSourcesPage,
+})
+
+const widgetsRoute = createRoute({
+  getParentRoute: () => rootRoute,
+  path: "/widgets",
+  component: WidgetsPage,
+})
+
+const layoutRoute = createRoute({
+  getParentRoute: () => rootRoute,
+  path: "/layout",
+  component: LayoutBuilderPage,
+})
+
+const presetsRoute = createRoute({
+  getParentRoute: () => rootRoute,
+  path: "/presets",
+  component: PresetsPage,
+})
+
+const routeTree = rootRoute.addChildren([
+  indexRoute,
+  dataSourcesRoute,
+  widgetsRoute,
+  layoutRoute,
+  presetsRoute,
+])
+
+export const router = createRouter({ routeTree })
+
+declare module "@tanstack/react-router" {
+  interface Register {
+    router: typeof router
+  }
+}