fix: remove duplicate main landmark and make calculator tabs arrow-operable

[Anexus5919]

📌 Related Issue

Fixes #892 (addresses A11Y-4 and A11Y-5)

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📝 Description

Two accessibility problems in FitnessCalculator.

🔹 What has been changed?

• A11Y-4 (duplicate main landmark): FitnessCalculator declared role="main" while rendering inside App's <main>, producing two main landmarks. Removed it (and the now-redundant aria-label).
• A11Y-5 (tabs not keyboard-operable): the tab bar already had the ARIA tab roles but no keyboard behavior. Implemented the WAI-ARIA tabs pattern: roving tabIndex (only the selected tab is tabbable) plus an onKeyDown handler for Arrow keys and Home/End that moves selection and focus.

🔹 Why are these changes needed?

• A page must have exactly one main landmark, and an ARIA tablist should be a single tab stop navigable with Arrow keys. The previous code had two main landmarks and tabs that weren't arrow-operable.

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🛠️ Type of Change

• 🐛 Bug Fix

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🧪 Testing

✅ Tests Performed

• Tested locally
• npx tsc --noEmit and npx eslint src/components/FitnessCalculator.tsx clean.
• Verified the roving-tabindex + Arrow/Home/End handler moves selection and focus, and that the duplicate role="main" is removed.

🔹 Note on automated tests

No committed test: keyboard/landmark behavior needs @testing-library/react, which can't currently run (@testing-library/dom evicted from node_modules).

🌐 Browsers Tested

Not applicable here (landmark/keyboard behavior; visible in the DevTools Accessibility pane).

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📷 Screenshots / Demo (if applicable)

Not applicable.

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📋 Checklist

• I have read the project's CONTRIBUTING guidelines
• My code follows the project style guidelines
• I have performed a self-review of my code
• I have tested my changes locally
• I have added/updated documentation where necessary (not applicable)
• My changes do not introduce new warnings or errors
• This PR is linked to an existing issue

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💬 Additional Notes

Branched from upstream/main; merges cleanly with the current base.

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[Anexus5919]

@Somil450 @diksha78dev Kindly have a review on this pr. Thanks!

[Anexus5919]

@Somil450 @diksha78dev Kindly have a review on this pr. Thanks!

[Anexus5919]

This failing check is from main, not this PR. Main's CI is currently broken (ShieldAlert used without importing it, plus untyped "privacy"/"terms&conditions" screens), so every PR goes red, even docs-only ones. Fix is up in #1039. Once it merges, a rebase turns this green. Merges cleanly with base.