chore(deps): update tailwindcss to v4

Switch to the @tailwindcss/vite plugin and the @import entry. Load the existing config with @config and keep class-based dark mode through a custom variant. Remove the postcss config and its autoprefixer dependency, which v4 no longer needs.

frontend/vite.config.ts

@@ -1,5 +1,6 @@
 import { defineConfig } from 'vite'
 import vue from '@vitejs/plugin-vue'
+import tailwindcss from '@tailwindcss/vite'
 import path from 'path'
 
 // Get backend URL from environment or use default
@@ -7,7 +8,7 @@ const BACKEND_URL = process.env.VITE_BACKEND_URL || 'http://localhost:8080'
 const FRONTEND_PORT = parseInt(process.env.VITE_PORT || '5173')
 
 export default defineConfig({
-  plugins: [vue()],
+  plugins: [vue(), tailwindcss()],
   resolve: {
     alias: {
       '@': path.resolve(__dirname, './src'),