feat: sync sumac/indigo changes into ulmo/indigo

[arbirali]

Summary

Brings all changes from sumac/indigo into ulmo/indigo as a single squash commit to maintain a clean history and pass commitlint.

Changes included

• Accessibility fixes for indigo theme/plugin based on audit
• Passing grade tooltip color contrast fix
• Focus visible thickness increase
• Sticky header reflow fix
• Theme toggle button focus styling
• Notification icon button fix in learning-mfe
• Alert-success styling fix for learning-mfe
• Dark theme improvements across multiple MFEs
• Misc styling and variable updates

Why squash?

The original sync branch (sync-sumac-into-ulmo) failed commitlint because historical commits in sumac/indigo did not follow the conventional commits format. Rather than rewriting dozens of historical commit
messages, the sync is delivered as a single conventional commit. The full sumac/indigo history remains intact on that branch.

Notes

• Replaces the previous sync-sumac-into-ulmo PR (branch deleted)
• All merge conflicts have been resolved
• 🤖 Generated with Claude Code