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-lock.json

@@ -12,6 +12,7 @@
         "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",
@@ -3280,7 +3281,6 @@
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       "dev": true,
       "license": "MIT",
-      "peer": true,
       "bin": {
         "jiti": "lib/jiti-cli.mjs"
       }
@@ -3381,7 +3381,6 @@
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       "license": "MPL-2.0",
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       "dependencies": {
         "detect-libc": "^2.0.3"
       },
@@ -3682,6 +3681,15 @@
         "yallist": "^3.0.2"
       }
     },
+    "node_modules/lucide-react": {
+      "version": "0.553.0",
+      "resolved": "https://registry.npmjs.org/lucide-react/-/lucide-react-0.553.0.tgz",
+      "integrity": "sha512-BRgX5zrWmNy/lkVAe0dXBgd7XQdZ3HTf+Hwe3c9WK6dqgnj9h+hxV+MDncM88xDWlCq27+TKvHGE70ViODNILw==",
+      "license": "ISC",
+      "peerDependencies": {
+        "react": "^16.5.1 || ^17.0.0 || ^18.0.0 || ^19.0.0"
+      }
+    },
     "node_modules/magic-string": {
       "version": "0.30.21",
       "resolved": "https://registry.npmjs.org/magic-string/-/magic-string-0.30.21.tgz",