refactor: Simplify UI styling and layout (#12)

* refactor: Simplify UI styling and layout

- Reduce CSS stylesheet from ~190 lines to ~50 lines
- Remove unused imports (QGraphicsDropShadowEffect, QPropertyAnimation, etc.)
- Simplify DropArea with cleaner styling
- Streamline layout with tighter spacing
- Keep button icons for visual clarity

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* refactor: Use class constants for FlatfieldDropArea styling

Consistent with main DropArea implementation.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: hongquanli