De-template the frontend and apply the design system

- FastAPI branding replaced throughout: title, favicon, logo (now the org/
  brand SVGs), footer links, route titles; workspace package renamed
- shadcn primitives restyled to the component radius/shadow map: pill
  controls, rounded-lg surfaces, shadow-e1/e2/e3
- floating frosted AppSidebar over a bg-card content region
- theme storage key aligned with the pre-paint script, MotionConfig added
- scripts/capture-screenshots.mjs backs the root 'make verify'

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/routes/_layout.tsx

@@ -22,9 +22,9 @@ export const Route = createFileRoute("/_layout")({
 
 function Layout() {
   return (
-    <SidebarProvider>
+    <SidebarProvider className="bg-card">
       <AppSidebar />
-      <SidebarInset>
+      <SidebarInset className="bg-card">
         <header className="sticky top-0 z-10 flex h-16 shrink-0 items-center gap-2 border-b px-4">
           <SidebarTrigger className="-ml-1 text-muted-foreground" />
         </header>