feat: modernize frontend UI with Tailwind v4 and professional design system

BREAKING CHANGE: Migrated to Tailwind CSS v4 configuration system

Key Changes:
- Migrated from Tailwind v3 to v4 configuration system
  - Removed tailwind.config.js (incompatible with v4)
  - Updated index.css with @theme directive and oklch color space
  - Defined all custom animations directly in CSS using @keyframes

- Redesigned LoginPage with modern, enterprise-grade UI:
  - Full-screen gradient background (blue → purple → pink)
  - Floating animated orbs with blur effects
  - Glass morphism white card with backdrop-blur
  - Gradient buttons with shadow effects
  - 7 custom animations: fade-in, slide-in-right, slide-in-left, scale-in, shimmer, pulse, float

- Added shadcn/ui components:
  - alert.tsx, dialog.tsx, input.tsx, label.tsx, select.tsx, tabs.tsx

- Updated dependencies:
  - Added class-variance-authority ^0.7.0
  - Added react-markdown ^9.0.1

- Updated frontend documentation:
  - Comprehensive README.md with feature list, tech stack, project structure
  - Quick start guide and deployment instructions

Technical Details:
- Tailwind v4 uses @import "tailwindcss" instead of @tailwind directives
- All theme customization now in @theme block with CSS variables
- Color system migrated to oklch for better perceptual uniformity
- Animation definitions moved from config to CSS @layer utilities

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/package.json

@@ -12,6 +12,7 @@
   "dependencies": {
     "@tanstack/react-query": "^5.90.7",
     "axios": "^1.13.2",
+    "class-variance-authority": "^0.7.0",
     "clsx": "^2.1.1",
     "i18next": "^25.6.2",
     "lucide-react": "^0.553.0",
@@ -19,6 +20,7 @@
     "react-dom": "^19.2.0",
     "react-dropzone": "^14.3.8",
     "react-i18next": "^16.3.0",
+    "react-markdown": "^9.0.1",
     "react-router-dom": "^7.9.5",
     "tailwind-merge": "^3.4.0",
     "zustand": "^5.0.8"