chore: Tailwind CSS v3 → v4 migration (#99)

* 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>

* 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>

* refactor: convert to OKLCH colors, move keyframes into @theme (Phase 3-4)

- Replaced all HSL color indirection with direct OKLCH values in @theme
- Moved all keyframes inside @theme block (v4 pattern)
- Eliminated hsl(var(--x)) double-indirection across 17 component files
- Replaced hsl() inline styles with var(--color-*) theme references
- Cleaned up redundant rdp-* utility blocks
- Fixed @custom-variant dark syntax to use :where()
- Added sidebar/glass/shadow vars as OKLCH in :root

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -21,6 +21,7 @@
     "@sentry/react": "^10.42.0",
     "@sentry/vite-plugin": "^5.1.1",
     "@stripe/stripe-js": "^8.7.0",
+    "@tailwindcss/vite": "^4.2.1",
     "@xyflow/react": "^12.10.0",
     "axios": "^1.13.4",
     "class-variance-authority": "^0.7.1",
@@ -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",