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/package.json

@@ -37,6 +37,7 @@
     "clsx": "^2.1.1",
     "form-data": "4.0.5",
     "lucide-react": "^0.562.0",
+    "motion": "^13.0.0",
     "next-themes": "^0.4.6",
     "react": "^19.1.1",
     "react-dom": "^19.2.3",