Enhance migration dependency graph with interactive features

Add several improvements to the interactive dependency graph:

- Display PR numbers as clickable links on selected nodes
- Add collapsible settings panel with gear icon for graph controls
- Implement CSS toggle switch for showing/hiding completed packages
- Add large graph warning (threshold: 1000 nodes) to prevent browser slowdown
- Improve node coloring based on migration category (bot-error, not-solvable, etc.)
- Use border highlighting instead of fill color for selected nodes
- Increase node padding for better readability

The settings panel consolidates graph direction, ranker algorithm,
alignment options, and the completed packages toggle into a
single floating panel accessed via gear icon.

Move inline styles to CSS module for better maintainability and
remove unnecessary comments from the codebase.

Author: Carreau