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/components/app-shell.tsx

@@ -0,0 +1,78 @@
+import { Link, useRouterState } from "@tanstack/react-router"
+import {
+  SidebarProvider,
+  Sidebar,
+  SidebarContent,
+  SidebarHeader,
+  SidebarMenu,
+  SidebarMenuItem,
+  SidebarMenuButton,
+  SidebarInset,
+  SidebarTrigger,
+} from "@/components/ui/sidebar"
+import { Separator } from "@/components/ui/separator"
+import { Toaster } from "@/components/ui/sonner"
+import {
+  LayoutDashboard,
+  Database,
+  Box,
+  Layers,
+  Save,
+} from "lucide-react"
+
+const NAV = [
+  { to: "/", label: "Dashboard", icon: LayoutDashboard },
+  { to: "/data-sources", label: "Data Sources", icon: Database },
+  { to: "/widgets", label: "Widgets", icon: Box },
+  { to: "/layout", label: "Layout", icon: Layers },
+  { to: "/presets", label: "Presets", icon: Save },
+] as const
+
+export function AppShell({ children }: { children: React.ReactNode }) {
+  const { location } = useRouterState()
+
+  return (
+    <SidebarProvider>
+      <Sidebar>
+        <SidebarHeader className="px-4 py-3">
+          <span className="text-lg font-bold tracking-tight">K-Frame</span>
+        </SidebarHeader>
+        <SidebarContent>
+          <SidebarMenu>
+            {NAV.map((item) => {
+              const active =
+                item.to === "/"
+                  ? location.pathname === "/"
+                  : location.pathname.startsWith(item.to)
+              return (
+                <SidebarMenuItem key={item.to}>
+                  <SidebarMenuButton asChild isActive={active}>
+                    <Link to={item.to}>
+                      <item.icon />
+                      <span>{item.label}</span>
+                    </Link>
+                  </SidebarMenuButton>
+                </SidebarMenuItem>
+              )
+            })}
+          </SidebarMenu>
+        </SidebarContent>
+      </Sidebar>
+      <SidebarInset>
+        <header className="flex h-12 items-center gap-2 border-b px-4">
+          <SidebarTrigger className="-ml-1" />
+          <Separator orientation="vertical" className="mr-2 h-4" />
+          <span className="text-muted-foreground text-sm">
+            {NAV.find((n) =>
+              n.to === "/"
+                ? location.pathname === "/"
+                : location.pathname.startsWith(n.to),
+            )?.label ?? ""}
+          </span>
+        </header>
+        <main className="flex-1 p-6">{children}</main>
+      </SidebarInset>
+      <Toaster />
+    </SidebarProvider>
+  )
+}