chore: switch from @tailwindcss/postcss to @tailwindcss/vite (Phase 2)

- Replaced @tailwindcss/postcss with @tailwindcss/vite plugin
- Deleted postcss.config.js (no longer needed)
- Tailwind now runs as a native Vite plugin for faster HMR

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

frontend/vite.config.ts

@@ -1,12 +1,14 @@
 /// <reference types="vitest/config" />
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
+import tailwindcss from '@tailwindcss/vite'
 import { sentryVitePlugin } from '@sentry/vite-plugin'
 import path from 'path'
 
 // https://vite.dev/config/
 export default defineConfig({
   plugins: [
+    tailwindcss(),
     react(),
     sentryVitePlugin({
       org: process.env.SENTRY_ORG,