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/components/Sidebar/AppSidebar.tsx

@@ -25,7 +25,13 @@ export function AppSidebar() {
     : baseItems
 
   return (
-    <Sidebar collapsible="icon">
+    // Floating frosted chrome over the bg-card content region; see the root
+    // DESIGN-GUIDELINES.md → Shells and → Overlay surfaces & content chips.
+    <Sidebar
+      collapsible="icon"
+      variant="floating"
+      className="[&>[data-sidebar=sidebar]]:bg-card/80 [&>[data-sidebar=sidebar]]:backdrop-blur-md [&>[data-sidebar=sidebar]]:shadow-e2"
+    >
       <SidebarHeader className="px-4 py-6 group-data-[collapsible=icon]:px-0 group-data-[collapsible=icon]:items-center">
         <Logo variant="responsive" />
       </SidebarHeader>