refactor: integrate drag-drop interface directly into main page

- Moved drag-drop upload interface to main homepage
- Maintains exact MUI styling and design system consistency
- Removed separate /app endpoint and public/index.html file
- Added tabbed interface (Upload File / Share Text) after hero section
- Integrated seamlessly with existing MUI components and color scheme
- All functionality preserved: file uploads, text sharing, one-time links
- Cleaner architecture with single entry point

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

Author: anoncam