fix: use UTC date parsing for contribution tooltips (JhaSourav07#1895)

## Description

Fixes timezone-related date shifting in contribution tooltips by
treating GitHub contribution dates as UTC calendar dates instead of
local timezone dates. The `formatDate` function previously used `new
Date(year, month - 1, day)` which caused negative UTC offsets (like
EST/PST) to display the previous day.

This PR switches to UTC-safe date parsing using ISO strings while
preserving the existing formatting behavior via the `Intl` API
explicitly set to UTC, and adds regression tests for multiple timezones.

Fixes JhaSourav07#1892 

## Pillar

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

## Visual Preview
*(No visual changes to the UI design, but the tooltips now accurately
display the correct date for users in negative UTC offsets).*

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [x] 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 (no raw elements, smooth animations, correct fonts).
- [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

components/InteractiveViewer.test.tsx

@@ -1,7 +1,7 @@
 import React from 'react';
 import { render, screen, fireEvent } from '@testing-library/react';
 import { describe, it, expect, beforeEach, vi } from 'vitest';
-import InteractiveViewer from './InteractiveViewer';
+import InteractiveViewer, { formatDate } from './InteractiveViewer';
 
 // getBoundingClientRect is not implemented in jsdom — mock it so mouse-position
 // tests can assert normalized values without relying on a real layout engine.
@@ -23,6 +23,23 @@ beforeEach(() => {
   Element.prototype.releasePointerCapture = vi.fn();
 });
 
+describe('formatDate', () => {
+  it('formats valid UTC date strings correctly', () => {
+    expect(formatDate('2025-06-15')).toBe('Jun 15, 2025');
+    expect(formatDate('2025-01-01')).toBe('Jan 1, 2025');
+    expect(formatDate('2025-12-31')).toBe('Dec 31, 2025');
+  });
+
+  it('returns empty string for empty input', () => {
+    expect(formatDate('')).toBe('');
+  });
+
+  it('returns original string for malformed input', () => {
+    expect(formatDate('2025-06')).toBe('2025-06');
+    expect(formatDate('invalid-date-string')).toBe('invalid-date-string');
+  });
+});
+
 describe('InteractiveViewer', () => {
   // ── Existing behaviour ────────────────────────────────────────────────────
 

components/InteractiveViewer.tsx

@@ -56,7 +56,7 @@ interface ActiveTooltipState {
   y: number;
 }
 
-const formatDate = (dateStr: string): string => {
+export const formatDate = (dateStr: string): string => {
   if (!dateStr) return '';
   const parts = dateStr.split('-');
   if (parts.length !== 3) return dateStr;
@@ -65,8 +65,10 @@ const formatDate = (dateStr: string): string => {
   const day = parseInt(parts[2], 10);
   if (isNaN(year) || isNaN(month) || isNaN(day)) return dateStr;
   try {
-    const date = new Date(year, month - 1, day);
+    const date = new Date(`${dateStr}T00:00:00Z`);
+    if (isNaN(date.getTime())) return dateStr;
     const formatted = date.toLocaleDateString('en-US', {
+      timeZone: 'UTC',
       month: 'short',
       day: 'numeric',
       year: 'numeric',