feat(frontend): scaffold Vite+React+TS+Tailwind/ShadCn + Vitest

TDD: App-renders-ThothII test RED then GREEN. ShadCn v4 (base-nova style)
init + button/checkbox/radio-group/textarea/card/dialog/badge/sonner added.
tailwind.config.ts extended with oklch CSS-variable color mapping required
by ShadCn v4 @apply directives. npm test passes; npm run build clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/utils.ts

@@ -0,0 +1,6 @@
+import { clsx, type ClassValue } from "clsx"
+import { twMerge } from "tailwind-merge"
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs))
+}