fix: clamp calorie deficit targets to a safe floor and guard BMI math

[Anexus5919]

📌 Related Issue

Fixes #872

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📝 Description

Two unguarded numeric paths in src/utils/fitnessCalculations.ts:

• Unsafe deficit targets (reachable): getCalorieRecommendations subtracted a flat 300/500 from TDEE with no lower bound, so smaller/sedentary users were shown deficit targets below the ~1200 kcal safe minimum (e.g. TDEE 1427 gives an aggressive deficit of 927), and these render in the Deficit tab.
• BMI Infinity/NaN (defensive): calculateBMI divided by height with no guard (Infinity for height 0, NaN for non-numeric input). Both current callers already validate height >= 100, so this is a robustness gap in the exported helper rather than a live UI bug.

🔹 What has been changed?

• Added MIN_SAFE_CALORIES = 1200 and clamped deficitMild/deficitAggressive to it; coerced a non-finite TDEE to 0.
• Guarded calculateBMI so non-finite weight or non-positive height yields 0, keeping the result and gauge finite.
• Added unit tests for both functions (src/utils/__tests__/fitnessCalculations.test.ts).

🔹 Why are these changes needed?

• Recommending a calorie target below the commonly cited ~1200 kcal/day minimum is a safety problem for smaller or sedentary users. Clamping to a floor keeps the recommendation safe, while surplus targets and normal-range deficits are unchanged.

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🛠️ Type of Change

• 🐛 Bug Fix

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🧪 Testing

✅ Tests Performed

• Tested locally
• npx vitest run src/utils/__tests__/fitnessCalculations.test.ts -> 6 tests pass.
• npx tsc --noEmit: changed file clean; npx eslint on the changed files: 0 problems.
• Verified surplus targets and normal-range (above-floor) deficits are unchanged.

🌐 Browsers Tested

Not applicable (pure calculation logic; verified via unit tests).

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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 latest upstream/main (5464425). The deficit clamp is the reachable safety fix; the calculateBMI guard is defensive (both current callers already validate height), framed as such above.

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[Anexus5919]

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