Add collapsible mobile navigation to CompetitionLayout

[coder13]

Motivation

• Improve navigation on small screens by providing a compact, accessible collapsible menu for competition pages.

Description

• Add useState and useMemo to manage isMobileMenuOpen and compute the currentTab, and ensure the mobile menu is closed on route changes via useEffect.
• Introduce a mobile-only header button that displays the current section and toggles a two-column grid of links, filtering out tabs with hiddenOnMobile, while retaining the existing desktop navigation markup and behavior.
• Adjust container and class layout to support responsive rendering without altering desktop navigation logic.

Testing

• Ran unit tests with yarn test and all tests passed.
• Built the app with yarn build and the build completed successfully.
• Ran linting with yarn lint and no new lint errors were reported.

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Codex Task

[netlify]

✅ Deploy Preview for competition-groups ready!

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Lighthouse	1 paths audited Performance: 65 Accessibility: 84 Best Practices: 100 SEO: 96 PWA: 90 View the detailed breakdown and full score reports

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