fix: wire the Welcome Plans button to the WorkoutPlansScreen

[Anexus5919]

📌 Related Issue

Fixes #883

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📝 Description

The Welcome "Plans" button called an empty onViewWorkoutPlans handler, and WorkoutPlansScreen (added by #708) existed but was never imported or rendered, with no Screen value, no transition, and no render block. The button did nothing and the screen was unreachable.

🔹 What has been changed?

• Added a workoutPlans screen to the Screen union and SCREEN_TRANSITIONS (reachable from welcome, returns to welcome).
• Lazy-loaded WorkoutPlansScreen and routed the button: onViewWorkoutPlans={() => navigateTo("workoutPlans")}.
• Rendered WorkoutPlansScreen with activePlan/setActivePlan state; exported ActivePlan from the component so App can type that state.

🔹 Why are these changes needed?

• The button and screen were both present but never connected, so a documented feature was unreachable. Wiring the route makes the screen reachable. The optional addXP/onStartWorkout props are intentionally omitted (onStartWorkout is declared but unused in the component, and useLeveling exposes no amount-based XP method), so they are left for a follow-up rather than faking a mapping.

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🛠️ Type of Change

• 🐛 Bug Fix

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🧪 Testing

✅ Tests Performed

• Tested locally
• npx tsc --noEmit and npx eslint clean for the changed files.
• Traced the navigation path: button -> navigateTo("workoutPlans") -> renders WorkoutPlansScreen -> back returns to welcome.

🔹 Note on automated tests

No committed test: App-level routing of a lazy-loaded screen is not covered by the repo's frontend test setup; verified by tsc/eslint and tracing the path.

🌐 Browsers Tested

Not applicable here (routing wiring; verified via static analysis).

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📷 Screenshots / Demo (if applicable)

A screenshot of the Welcome screen's "Plans" button could be added to show the entry point.

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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.