feat(deps): migrate Material UI v4 to MUI v5

Context

Renovate PR #409 tried to replace @material-ui/core with @mui/material, but this repository still uses the Material UI v4 package family throughout the codebase.

A package-only replacement is unsafe because the app still imports @material-ui/core, and related v4 packages remain in use:

• @material-ui/core
• @material-ui/icons
• @material-ui/lab
• @material-ui/styles
• gatsby-plugin-material-ui

Scope

• Audit all Material UI v4 imports and theme usage.
• Plan the migration path to MUI v5 equivalents.
• Update package dependencies as a coordinated migration, not a Renovate-only package swap.
• Update Gatsby integration for MUI v5.
• Run unit tests and a local build/dev smoke test.

Acceptance Criteria

• No imports from @material-ui/* remain unless intentionally retained and documented.
• MUI v5 packages are installed and compatible with the app's React/Gatsby stack.
• Theme/provider setup works with MUI v5.
• npm test passes.
• A build/dev smoke test passes or any blocker is documented in TROUBLESHOOTING.md.

Related

• Blocks #409

Start by auditing the repository for imports from the listed @material-ui packages, theme/provider setup, and Gatsby integration. Review the dependency changes related to Renovate PR #409, then run npm test and a local build/dev smoke test. Done means MUI v5 is installed compatibly, no unintended v4 imports remain, tests pass, and any smoke-test blocker is documented in TROUBLESHOOTING.md.