chore: run Tailwind v4 upgrade tool (Phase 1)

- Upgraded tailwindcss v3 → v4.2.1, postcss plugin to @tailwindcss/postcss
- Deleted tailwind.config.js, migrated theme to CSS @theme block in index.css
- Replaced @tailwind directives with @import 'tailwindcss'
- Added @custom-variant dark, @utility blocks for custom utilities
- Updated class names across 128 files (shadow-sm → shadow-xs, etc.)
- Removed autoprefixer (built into v4)
- Added migration plan doc

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

frontend/package.json

@@ -43,6 +43,7 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.39.1",
+    "@tailwindcss/postcss": "^4.2.1",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.2",
     "@testing-library/user-event": "^14.6.1",
@@ -50,14 +51,13 @@
     "@types/react": "^19.2.5",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^5.1.1",
-    "autoprefixer": "^10.4.23",
     "eslint": "^9.39.1",
     "eslint-plugin-react-hooks": "^7.0.1",
     "eslint-plugin-react-refresh": "^0.4.24",
     "globals": "^16.5.0",
     "jsdom": "^28.0.0",
     "postcss": "^8.5.6",
-    "tailwindcss": "^3.4.19",
+    "tailwindcss": "^4.2.1",
     "typescript": "~5.9.3",
     "typescript-eslint": "^8.46.4",
     "vite": "^7.2.4",