Add island right-dock feature

[Microck]

Description

Add ability to dock the island panel to the right side of the screen.

Type of Change

• New feature (non-breaking change which adds functionality)

Changes Made

• Drag island to right edge to dock as fixed panel
• Visual drop-zone indicator while dragging
• Persist docked state across sessions
• Undock by dragging away from right edge
• Right-docked island fills available vertical space
• Custom scrollbar styling for layers and side panel

Testing

• Tested in Firefox

Test Configuration:

• OS: Windows 10
• Browser: Firefox

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have tested my changes thoroughly

Related Issues

N/A