Reorganize UI: move file selection above tabs for better UX

- Move file browsing to shared section at top of window
- Tabs (Convert, Split, Merge) now appear below file selection
- Users can switch between operations without re-browsing for files
- Centralize file selection logic in browse_main_files() method
- Remove redundant file browsing UI from individual tabs
- Update tab displays to show relevant file information
- Tab-aware browse button text changes based on active operation

This improves the workflow by eliminating redundant file selection
steps when switching between different operations.

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

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

Author: alexander-vyh