fix: repair the broken main build (ShieldAlert import, legal screens)

[Anexus5919]

📌 Related Issue

Fixes #1038

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📝 Description

The frontend build is currently broken on main (CI has failed on the last several commits, and every open PR inherits the failure). Two independent regressions are responsible.

🔹 What has been changed?

• ShieldAlert import: WorkoutScreen.tsx renders <ShieldAlert /> but never imported it, so the lint step fails with react/jsx-no-undef. Added ShieldAlert to the existing lucide-react import.
• Legal screens typing: App.tsx compares currentScreen against "privacy" and "terms&conditions" and renders both screens (and WelcomeScreen navigates to them), but neither was a member of the Screen union, so tsc failed with TS2367. Added both to the Screen union, added their SCREEN_TRANSITIONS entries (["welcome"]), and allowed them as transitions from welcome.

🔹 Why are these changes needed?

• A used-but-unimported component breaks lint, and comparing currentScreen to string literals outside its type breaks the type-check. Together they fail CI for every branch. These two small fixes make main build and lint cleanly again so open PRs can go green.

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🛠️ Type of Change

• 🐛 Bug Fix

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🧪 Testing

✅ Tests Performed

• Tested locally
• npx tsc --noEmit is clean (with dependencies installed, matching CI); the previously failing TS2367 errors on App.tsx are gone.
• npx eslint on the changed files is clean; the react/jsx-no-undef error on WorkoutScreen.tsx:1035 is resolved.
• Verified welcome can transition to privacy / terms&conditions and each returns to welcome.

🌐 Browsers Tested

Not applicable (build/type fix; no runtime behavior change beyond the legal-screen navigation already present).

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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. This unblocks CI for the other open PRs, which are currently red only because of this main breakage (a Markdown-only PR fails at the same lint step).

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[Anexus5919]

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