Redesign notebook UI with gutter ribbon layout

[rgbkrk]

Summary

Completely redesigned the notebook interface to feel cleaner and more like Google Colab. Replaced the stocky card design with a paper-like aesthetic featuring a continuous left-side gutter ribbon.

Key Changes

• Gutter ribbon system: Each cell displays a thin colored ribbon on the left that changes color by cell type (gray for code, amber for markdown)
• Smart play button: Positioned in the gutter action area, visible when focused or on cell hover, hidden otherwise
• Simplified cell design: Removed execution metadata lines ("Executed") for a more focused interface
• Keyboard navigation: Added smart cell navigation with Shift-Enter creating new cells at the end
• Duplicate output fix: Fixed React StrictMode causing duplicate outputs via callback ref pattern

Testing

• Play button appears/disappears correctly based on focus state
• Gutter ribbon color changes per cell type
• Keyboard navigation works between cells
• New cells created automatically when needed