Base converter implementation

• Inspect the existing GB opcodes page and choose the shared visual patterns.
• Build the decimal/hexadecimal/binary converter markup and interactions.
• Add responsive styling, formatting states, and accessible input labels.
• Verify conversions and layout locally.
• Commit the completed feature.

Main site tools navigation

• Add an accessible Tools dropdown to the shared Jekyll navigation.
• Style the dropdown for desktop and mobile layouts.
• Verify both tool links and keyboard behavior.
• Commit and push the navigation update.