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>

.gitignore

@@ -15,3 +15,6 @@ __pycache__/
 
 # Rendered pipeline graphs (matplotlib output from backend/app/flow).
 pipeline.png
+
+# Output of `make verify` (root Makefile).
+/frontend/screenshots/