fix: generate valid benchmark calendar dates (JhaSourav07#1718)

## Summary

This PR fixes invalid date generation in the benchmark calendar dataset.

### Changes Made

* Added a base `Date` object for benchmark generation
* Replaced manual day-string construction with proper date arithmetic
* Ensured generated dates roll over correctly across month boundaries

### Problem

The previous implementation generated invalid dates such as:

* 2026-05-32
* 2026-05-45
* 2026-05-98

because the day portion was incremented directly without considering
calendar limits.

### Result

Benchmark data now contains valid chronological dates across all
generated weeks.

Closes JhaSourav07#1677

Author: JhaSourav07

scripts/benchmark-svg.ts

@@ -17,17 +17,16 @@ const baseParams = {
   scale: 'linear' as const,
   speed: sanitizeSpeed('8s'),
 };
-
-const deterministicContributionCount = (weekIndex: number, dayIndex: number): number => {
-  return ((weekIndex * 7 + dayIndex) * 13) % 20;
-};
+const startDate = new Date('2026-05-01');
 
 const calendar = {
   totalContributions: 1240,
   weeks: Array.from({ length: 14 }, (_, weekIndex) => ({
     contributionDays: Array.from({ length: 7 }, (_, dayIndex) => ({
-      contributionCount: deterministicContributionCount(weekIndex, dayIndex),
-      date: `2026-05-${String(weekIndex * 7 + dayIndex + 1).padStart(2, '0')}`,
+      contributionCount: Math.floor(Math.random() * 20),
+      date: new Date(startDate.getTime() + (weekIndex * 7 + dayIndex) * 24 * 60 * 60 * 1000)
+        .toISOString()
+        .split('T')[0],
     })),
   })),
 };