refactor(calculate): extract isStreakAlive helper (JhaSourav07#761)

## Description

Fixes JhaSourav07#304 

Extracted the grace-period streak logic into a dedicated
`isStreakAlive()` helper to improve readability and testability in
`calculateStreak`.

## Changes made

- Added and exported `isStreakAlive()` helper
- Replaced inline grace-period boolean logic in `calculateStreak`
- Added dedicated helper tests covering:
  - both active
  - only today active
  - only yesterday active
  - both inactive
  - null yesterday edge case

## Pillar

- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview

N/A (logic refactor and test changes)

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally.
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors.
- [x] My commits follow the Conventional Commits format.
- [ ] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard.
- [x] I joined the CommitPulse Discord community for contributor
discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/calculate.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { calculateStreak, calculateMonthlyStats } from './calculate';
+import { calculateStreak, calculateMonthlyStats, isStreakAlive } from './calculate';
 import type { ContributionCalendar } from '../types';
 
 // Turns a flat array of daily counts into the ContributionCalendar shape,
@@ -289,6 +289,28 @@ describe('calculateStreak — timezone awareness', () => {
   });
 });
 
+describe('isStreakAlive', () => {
+  it('returns true when both today and yesterday have contributions', () => {
+    expect(isStreakAlive({ contributionCount: 1 }, { contributionCount: 1 })).toBe(true);
+  });
+
+  it('returns true when only today has contributions', () => {
+    expect(isStreakAlive({ contributionCount: 1 }, { contributionCount: 0 })).toBe(true);
+  });
+
+  it('returns true when only yesterday has contributions', () => {
+    expect(isStreakAlive({ contributionCount: 0 }, { contributionCount: 1 })).toBe(true);
+  });
+
+  it('returns false when both today and yesterday have zero contributions', () => {
+    expect(isStreakAlive({ contributionCount: 0 }, { contributionCount: 0 })).toBe(false);
+  });
+
+  it('returns false when yesterday is null and today has no contributions', () => {
+    expect(isStreakAlive({ contributionCount: 0 }, null)).toBe(false);
+  });
+});
+
 describe('calculateMonthlyStats', () => {
   it('calculates monthly stats correctly when both months have commits', () => {
     const calendar = {

lib/calculate.ts

@@ -50,6 +50,12 @@ const stats = calculateStreak(
   new Date('2026-05-25T10:00:00Z')
 );
 */
+export function isStreakAlive(
+  today: { contributionCount: number },
+  yesterday: { contributionCount: number } | null
+): boolean {
+  return today.contributionCount > 0 || (yesterday?.contributionCount ?? 0) > 0;
+}
 
 export function calculateStreak(
   calendar: ContributionCalendar,