Build and apply a shared design system audit across the workspace

[MaggieAppleton]

Summary

• Add a design-audit route covering foundations, surfaces, controls, authored content, and icon specimens.
• Introduce shared icon assets and update navigation, chat, workspace, repository, and document-picker UI.
• Refine editor, research, decision, planner, and theme styling around shared design tokens.
• Add the @chopin/icons package and update workspace dependencies.
• Expand coverage for design-audit pages, navigation, icons, themes, tokens, chat transcripts, and editor behavior.

Testing

• Added and updated unit/component tests across the affected web and editor surfaces.
• Not run (not requested).