fix: migrate Souls Studio from Tailwind v3 to v4

Tailwind v4 moved the PostCSS plugin to @tailwindcss/postcss and removed
@tailwind directives in favor of @import "tailwindcss". This fixes the
studio build failure after the dependency update.

- Replace tailwindcss with @tailwindcss/postcss in PostCSS config
- Update index.css to use @import "tailwindcss" instead of @tailwind directives
- Remove tailwind.config.js (auto-detected in v4)
- Remove autoprefixer (built into Tailwind v4)

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

Author: thedaviddias

apps/souls-studio/package.json

@@ -57,13 +57,13 @@
   "devDependencies": {
     "@playwright/test": "^1.58.2",
     "@sentry/vite-plugin": "^5.1.1",
+    "@tailwindcss/postcss": "^4",
     "@tauri-apps/cli": "^2.10.1",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.2",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^6.0.1",
-    "autoprefixer": "^10.4.27",
     "jsdom": "^29.0.0",
     "postcss": "^8.5.8",
     "tailwindcss": "^4.2.1",

apps/souls-studio/postcss.config.js

@@ -1,6 +1,5 @@
 export default {
   plugins: {
-    tailwindcss: {},
-    autoprefixer: {},
+    '@tailwindcss/postcss': {},
   },
 }

apps/souls-studio/src/index.css

@@ -1,6 +1,4 @@
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
+@import "tailwindcss";
 
 :root {
   --bg-primary: #f8f8f8;