feat(ui): F0 — Tailwind + design tokens + theme infrastructure

Install Tailwind v3, Radix primitives, cva/tailwind-merge/clsx, lucide-react.
Add HSL design tokens (light + dark) with the Indigo #5b5bf0 accent isolated
to a single --primary token (swappable for a future accent picker), darkMode
'class', anti-flash inline script, useTheme hook, and cn() helper. styles.css
still loads for not-yet-migrated pages. Build green.

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

frontend/postcss.config.js

@@ -0,0 +1,6 @@
+export default {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+};