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/vite.config.ts

@@ -0,0 +1,22 @@
+import path from "path"
+import tailwindcss from "@tailwindcss/vite"
+import react from "@vitejs/plugin-react"
+import { defineConfig } from "vite"
+
+// https://vite.dev/config/
+export default defineConfig({
+  plugins: [react(), tailwindcss()],
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "./src"),
+    },
+  },
+  server: {
+    proxy: {
+      "/api": {
+        target: "http://localhost:3000",
+        changeOrigin: true,
+      },
+    },
+  },
+})