Improve profile tab accessibility

[alan747271363-art]

Summary

• add tablist/tab/tabpanel semantics to the profile tabs used for Workspaces, Badges, Bounties, and Assigned Bounties
• link tabs and panels with stable ids via aria-controls/aria-labelledby
• support Enter, Space, ArrowLeft, and ArrowRight keyboard interactions
• add regression coverage for selected tab state, tab/panel linkage, and keyboard navigation

Closes #1234

Validation

• npx.cmd --yes prettier@3.0.3 --config .prettierrc.json --check src/pages/people/tabs/index.tsx src/pages/people/tabs/tests/index.spec.tsx
• cmd /c "set REACT_APP_IS_TEST=true&& set NODE_ENV=test&& node node_modules\jest\bin\jest.js --runTestsByPath src/pages/people/tabs/tests/index.spec.tsx --runInBand --no-coverage --silent --transformIgnorePatterns ./svg/"
• npx.cmd eslint src/pages/people/tabs/index.tsx src/pages/people/tabs/tests/index.spec.tsx --max-warnings 100 (passes with one existing react-hooks/exhaustive-deps warning in index.tsx)
• git diff --check