fix(a11y): add keyboard focus trap for Help sidebar and lightbox

[Innagl]

Describe your changes

Introduce overlay accessibility utilities for the Help sidebar, TabBar, and user guide image lightbox, yielding Tab/Esc to nested MUI modals.

Write your issue number after "Fixes "

Fixes #4225

Please ensure all items are checked off before requesting a review:

• I deployed the code locally.
• I have performed a self-review of my code.
• I have included the issue # in the PR.
• I have labelled the PR correctly.
• The issue I am working on is assigned to me.
• I have avoided using hardcoded values to ensure scalability and maintain consistency across the application.
• I have ensured that font sizes, color choices, and other UI elements are referenced from the theme.
• My pull request is focused and addresses a single, specific feature.
• If there are UI changes, I have attached a screenshot or video to this PR.
• If I added or modified an API endpoint, the change is reflected in the generated OpenAPI spec (npm run generate:swagger).
• If the endpoint requires authentication, it uses authenticateJWT and the generated spec declares bearerAuth security.
• I ran npm run check:api-drift and committed the regenerated swagger.yaml and endpoints.ts.
• If this PR adds or modifies an organization-scoped table, the tenant isolation registry and test matrix are updated. See the tenant isolation runbook for details.