feat: modernize frontend architecture with professional UI/UX design

Complete redesign of frontend interface with focus on usability, visual hierarchy, and professional appearance:

**Design System:**
- Implemented clean blue color theme (#3B82F6) with professional palette
- Created consistent spacing, shadows, and typography system
- Added reusable utility classes (page-header, section, status-badge-*)
- Removed excessive gradients and decorative effects

**Layout Architecture:**
- Redesigned main layout with 256px sidebar navigation
- Sidebar includes logo, navigation with descriptions, and user profile
- Main content area with search bar and scrollable content
- Replaced horizontal navigation with vertical sidebar pattern

**Page Redesigns:**
1. LoginPage: Split-screen design with branding (left) and clean form (right)
   - Feature highlights with icons and statistics
   - Mobile responsive design
   - Professional gradient background with subtle pattern

2. UploadPage: Added 3-step visual progress indicator
   - Better file organization with summary and status badges
   - Clear action bar with confirmation message
   - Improved file list presentation

3. ProcessingPage: Enhanced progress visualization
   - Large progress bar with percentage display
   - 4-column stats grid (Completed, Processing, Failed, Total)
   - Clean file status list with processing times

4. ResultsPage: Improved 5-column layout (2 for list, 3 for preview)
   - Added stats cards for accuracy, processing time, and text blocks
   - Better preview panel with detailed metrics
   - Export and translate action buttons

5. ExportPage: Better organization with 2-column layout
   - Visual format selection with icons (TXT, JSON, Excel, Markdown, PDF)
   - Improved form controls and option organization
   - Sticky preview sidebar showing current configuration

**Component Updates:**
- Updated Button component with proper variants
- Enhanced Card component with hover effects
- Maintained FileUpload component functionality
- Added lucide-react for modern iconography

**Technical Improvements:**
- Fixed Tailwind CSS v4 compatibility issues with @apply
- Removed decorative animations in favor of functional ones
- Improved accessibility with proper labels and ARIA attributes
- Better color contrast and readability

This redesign transforms the interface from a basic layout to a professional, enterprise-ready application with clear visual hierarchy and excellent usability.

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

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

frontend/package.json

@@ -14,6 +14,7 @@
     "axios": "^1.13.2",
     "clsx": "^2.1.1",
     "i18next": "^25.6.2",
+    "lucide-react": "^0.553.0",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
     "react-dropzone": "^14.3.8",