fix: return null deltaPercentage when previous month has zero contributions

[KaparthyReddy]

Fixes #908

Description

calculateMonthlyStats in lib/calculate.ts hardcoded deltaPercentage = 100 whenever previousMonthTotal === 0, regardless of how many contributions were made this month. This is mathematically undefined — dividing by zero has no meaningful percentage result. A user with 1 commit vs 0 last month and a user with 500 commits vs 0 last month both saw +100%, which is misleading and factually incorrect.

Changes:

• deltaPercentage is now typed as number | null in MonthlyStats (types/index.ts)
• calculateMonthlyStats returns null when previousMonthTotal === 0 instead of hardcoded 100
• generateMonthlySVG and generateAutoThemeMonthlySVG now render N/A when deltaPercentage is null
• Updated existing test in lib/calculate.test.ts to expect null instead of 100

Files changed: lib/calculate.ts, lib/svg/generator.ts, types/index.ts, lib/calculate.test.ts

Pillar

• 🛠️ Other (Bug fix, refactoring, docs)

Visual Preview

N/A — delta indicator will now show N/A instead of +100% for users with zero contributions last month.

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.
• I have updated README.md if I added a new theme or URL parameter.
• I have starred the repo.
• I have made sure that I have only one commit to merge in this PR.
• The SVG output matches the CommitPulse "premium quality" aesthetic standard.
• (Recommended) I joined the CommitPulse Discord community.

[github-actions]

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[github-actions]

🎉 Congratulations @KaparthyReddy! Your PR has been successfully merged. 🚀

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