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/package.json

@@ -0,0 +1,57 @@
+{
+  "name": "spa",
+  "private": true,
+  "version": "0.0.1",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b && vite build",
+    "lint": "eslint .",
+    "format": "prettier --write \"**/*.{ts,tsx}\"",
+    "typecheck": "tsc --noEmit",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "@base-ui/react": "^1.6.0",
+    "@fontsource-variable/geist": "^5.2.9",
+    "@tailwindcss/vite": "^4",
+    "@tanstack/react-query": "^5.101.0",
+    "@tanstack/react-router": "^1.170.16",
+    "class-variance-authority": "^0.7.1",
+    "clsx": "^2.1.1",
+    "cmdk": "^1.1.1",
+    "date-fns": "^4.4.0",
+    "embla-carousel-react": "^8.6.0",
+    "input-otp": "^1.4.2",
+    "lucide-react": "^1.21.0",
+    "next-themes": "^0.4.6",
+    "radix-ui": "^1.6.0",
+    "react": "^19.2.6",
+    "react-day-picker": "^10.0.1",
+    "react-dom": "^19.2.6",
+    "react-resizable-panels": "^4.11.2",
+    "recharts": "3.8.0",
+    "shadcn": "^4.11.0",
+    "sonner": "^2.0.7",
+    "tailwind-merge": "^3.6.0",
+    "tailwindcss": "^4",
+    "tw-animate-css": "^1.4.0",
+    "vaul": "^1.1.2"
+  },
+  "devDependencies": {
+    "@eslint/js": "^10",
+    "@types/node": "^24",
+    "@types/react": "^19",
+    "@types/react-dom": "^19",
+    "@vitejs/plugin-react": "^6",
+    "eslint": "^10",
+    "eslint-plugin-react-hooks": "^7.1.1",
+    "eslint-plugin-react-refresh": "^0.5.2",
+    "globals": "^17",
+    "prettier": "^3.8.3",
+    "prettier-plugin-tailwindcss": "^0.8.0",
+    "typescript": "~6",
+    "typescript-eslint": "^8",
+    "vite": "^8"
+  }
+}