test(svg): add constants integrity and theme safety tests (JhaSourav07#3092)

## Description

Fixes JhaSourav07#2885

Adds unit tests for SVG constants to ensure integrity of layout values,
scaling logic, font mappings, and milestone ordering. These tests help
prevent regressions in SVG rendering and ensure consistent behavior
across layout scaling and rendering logic.

## Pillar

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

## Visual Preview

No UI changes. This PR only adds unit tests for SVG constants and
validation logic.

## 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): ...`, `test(svg): ...`).
- [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).
- [x] I have joined the CommitPulse Discord community for contributor
discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/svg/constants.theme-contrast.test.ts

@@ -0,0 +1,46 @@
+import {
+  SVG_WIDTH,
+  SVG_HEIGHT,
+  GHOST_HEIGHT_PX,
+  LOG_SCALE_MULTIPLIER,
+  LINEAR_SCALE_MULTIPLIER,
+  MAX_LOG_HEIGHT,
+  MAX_LINEAR_HEIGHT,
+  FONT_MAP,
+  CONTRIBUTION_MILESTONES,
+  STREAK_MILESTONES,
+} from './constants';
+
+import { describe, it, expect } from 'vitest';
+
+describe('SVG constants integrity', () => {
+  it('should have valid SVG dimensions', () => {
+    expect(SVG_WIDTH).toBe(600);
+    expect(SVG_HEIGHT).toBe(420);
+  });
+  it('should keep ghost height within safe rendering range', () => {
+    expect(GHOST_HEIGHT_PX).toBeGreaterThan(0);
+    expect(GHOST_HEIGHT_PX).toBeLessThan(10);
+  });
+  it('should ensure log scale multiplier is larger than linear scale multiplier', () => {
+    expect(LOG_SCALE_MULTIPLIER).toBeGreaterThan(LINEAR_SCALE_MULTIPLIER);
+    expect(LINEAR_SCALE_MULTIPLIER).toBeGreaterThan(0);
+  });
+  it('should contain valid font mappings', () => {
+    expect(FONT_MAP.jetbrains).toContain('monospace');
+    expect(FONT_MAP.fira).toContain('monospace');
+    expect(FONT_MAP.roboto).toContain('sans-serif');
+
+    expect(Object.keys(FONT_MAP).length).toBeGreaterThan(0);
+  });
+  it('should have correctly ordered contribution milestones', () => {
+    expect(CONTRIBUTION_MILESTONES).toEqual([...CONTRIBUTION_MILESTONES].sort((a, b) => a - b));
+
+    expect(CONTRIBUTION_MILESTONES[0]).toBe(1);
+  });
+  it('should have valid streak milestones in ascending order', () => {
+    expect(STREAK_MILESTONES).toEqual([...STREAK_MILESTONES].sort((a, b) => a - b));
+
+    expect(STREAK_MILESTONES[0]).toBe(3);
+  });
+});