app: shell + scificn-ui frontend (SCI-FI theme, brightened --text-muted)

scificn-ui v0.1.0 integrated copy-paste per its docs: Button/Panel/Badge/Card/Alert
in components/ui/, cn() helper in lib/utils.ts, theme tokens in app/globals.css.
Deliberate deviation from upstream: --text-muted #4A4A40 -> #6B6B5C (+12pt lightness)
for readability on the near-black background (default SCI-FI theme only). Landing page
proves the integration; no game UI yet.

lib/utils.ts

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