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/signup.tsx

@@ -51,7 +51,7 @@ export const Route = createFileRoute("/signup")({
   head: () => ({
     meta: [
       {
-        title: "Sign Up - FastAPI Cloud",
+        title: "Sign Up - Fluksio",
       },
     ],
   }),