refactor(calculate): extract safe percentage rounding logic into a single reusable helper function

[ChetanSenta]

Description

Fixes #6148

Pillar

• 🎨 Pillar 1 — New Theme Design
• 📐 Pillar 2 — Geometric SVG Improvement
• 🕐 Pillar 3 — Timezone Logic Optimization
• 🛠️ Other (Refactoring)

Visual Preview

Zero behavioral or visual outcome changes. All computed statistical outputs remain identical to previous calculations.

What this PR does

Cleans up redundant, duplicated code patterns in lib/calculate.ts. The codebase contained several identical inline ternary calculations (total === 0 ? 0 : Math.round(...)) designed to prevent NaN or zero division errors when computing ratios like weekendRatio.

This refactor bundles this pattern into a robust calculateSafePercentage() utility function, improving codebase maintainability and readability.

Changes

File	Change
lib/calculate.ts	Created calculateSafePercentage() helper and replaced redundant inline ternaries.
lib/calculate.test.ts	Added regression tests verifying zero-division protection and accurate rounding precision.

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally.
• I have run npm run format and npm run lint locally and resolved all errors.
• My commits follow the Conventional Commits format.
• My branch has only one clean commit to merge.

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📦 Next.js Bundle Size Report (Gzipped Sizes)

✨ No significant bundle size changes detected.

📊 Summary of Totals

Category	PR Size	Base Size	Difference
Total JS	3694.34 KB	3694.34 KB	0 B
Total CSS	296.06 KB	296.06 KB	0 B