test: add timezone boundary tests for streak PNG route

[AMAN194701]

Description

Closes #6790

Added a dedicated timezone boundary test suite for the Streak PNG route.

Changes

• Added app/api/streak/png/route.timezone-boundaries.test.ts
• Implemented 5 test cases covering:
  • Standard timezone settings (UTC, EST, IST, JST)
  • Calendar date alignment across timezones
  • Leap year boundary validation
  • Calendar date formatting utility verification
  • Daylight Saving Time (DST) transition handling
• Verified all tests pass successfully using Vitest.

Pillar

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

Visual Preview

N/A (Test-only changes)

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• My commits follow the Conventional Commits format (e.g., feat(themes): ..., fix(calculate): ...).
• 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.

[github-actions]

🚨 Hey @AMAN194701, the CI Pipeline is failing on this PR and it has been marked as status:blocked.

Please fix the issues before this can be reviewed. Here's how:

1. Run checks locally before pushing:

npm run format:check   # Check Prettier formatting
npm run lint           # Run ESLint
npm run typecheck      # TypeScript type check
npm run test           # Run unit tests (Vitest)
npm run build          # Verify production build passes

2. Auto-fix common issues:

npm run format         # Auto-fix formatting with Prettier
npm run lint -- --fix  # Auto-fix lint errors where possible

3. Check the full failure log here:
👉 View CI Run

Once you push a fix and the CI passes, the status:blocked label will be removed automatically. 💪