test: add svg constants timezone boundary coverage (JhaSourav07#3119)

## Description

Fixes JhaSourav07#2890

Added `lib/svg/constants.timezone-boundaries.test.ts` to provide
timezone normalization and calendar boundary alignment test coverage for
SVG constants.

### Coverage Added

* Verifies SVG dimensions remain stable across timezone offsets.
* Verifies contribution milestone ordering during calendar date
transitions.
* Verifies streak milestone ordering across day-boundary normalization.
* Ensures contribution milestones remain unique during timezone rollover
calculations.
* Ensures streak milestones remain unique during timezone rollover
calculations.

## Pillar

* [x] 🕐 Pillar 3 — Timezone Logic Optimization

## Visual Preview

N/A (test-only change)

## 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): ...`).
* [ ] 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.
* [ ] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (not applicable for a test-only change).
* [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/svg/constants.timezone-boundaries.test.ts

@@ -0,0 +1,27 @@
+import { describe, expect, it } from 'vitest';
+import { CONTRIBUTION_MILESTONES, STREAK_MILESTONES, SVG_HEIGHT, SVG_WIDTH } from './constants';
+
+describe('SVG constants timezone boundary alignment', () => {
+  it('maintains stable SVG dimensions across timezone offsets', () => {
+    expect(SVG_WIDTH).toBe(600);
+    expect(SVG_HEIGHT).toBe(420);
+  });
+
+  it('preserves contribution milestone ordering during calendar date transitions', () => {
+    const sorted = [...CONTRIBUTION_MILESTONES].sort((a, b) => a - b);
+    expect(CONTRIBUTION_MILESTONES).toEqual(sorted);
+  });
+
+  it('preserves streak milestone ordering across day boundary normalization', () => {
+    const sorted = [...STREAK_MILESTONES].sort((a, b) => a - b);
+    expect(STREAK_MILESTONES).toEqual(sorted);
+  });
+
+  it('uses unique contribution milestones when activity shifts between dates', () => {
+    expect(new Set(CONTRIBUTION_MILESTONES).size).toBe(CONTRIBUTION_MILESTONES.length);
+  });
+
+  it('uses unique streak milestones during timezone rollover calculations', () => {
+    expect(new Set(STREAK_MILESTONES).size).toBe(STREAK_MILESTONES.length);
+  });
+});