test(svg-constants): add coverage for SVG geometry and configuration constants (JhaSourav07#3137)

## Description
This PR adds a dedicated test suite for lib/svg/constants.ts to validate
SVG geometry, scaling configuration, font mappings, and milestone
definitions.

## Test Coverage

The new test suite includes 5 test cases:

- Verifies SVG viewport dimensions are positive integers.
- Validates scaling multipliers and height configuration values.
- Ensures font mappings are correctly defined and non-empty.
- Confirms contribution and streak milestone arrays remain sorted and
unchanged.
- Protects against accidental modification of critical SVG constant
values.

Fixes JhaSourav07#2291 

## Pillar


- [x] 📐 Pillar 2 — Geometric SVG Improvement
- [x] 🛠️ Other (Bug fix, refactoring, docs)


## 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).
- [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/svg/constants.test.ts

@@ -0,0 +1,66 @@
+import { describe, it, expect } from 'vitest';
+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';
+
+describe('svg constants', () => {
+  it('exports positive SVG dimensions', () => {
+    expect(SVG_WIDTH).toBeGreaterThan(0);
+    expect(SVG_HEIGHT).toBeGreaterThan(0);
+    expect(Number.isInteger(SVG_WIDTH)).toBe(true);
+    expect(Number.isInteger(SVG_HEIGHT)).toBe(true);
+  });
+
+  it('exports valid scaling and height values', () => {
+    expect(GHOST_HEIGHT_PX).toBeGreaterThan(0);
+
+    expect(LOG_SCALE_MULTIPLIER).toBeGreaterThan(0);
+    expect(LINEAR_SCALE_MULTIPLIER).toBeGreaterThan(0);
+
+    expect(MAX_LOG_HEIGHT).toBeGreaterThan(0);
+    expect(MAX_LINEAR_HEIGHT).toBeGreaterThan(0);
+
+    expect(MAX_LOG_HEIGHT).toBeLessThanOrEqual(SVG_HEIGHT);
+    expect(MAX_LINEAR_HEIGHT).toBeLessThanOrEqual(SVG_HEIGHT);
+  });
+
+  it('exports valid font mappings', () => {
+    expect(Object.keys(FONT_MAP)).toEqual(['jetbrains', 'fira', 'roboto']);
+
+    Object.values(FONT_MAP).forEach((font) => {
+      expect(typeof font).toBe('string');
+      expect(font.length).toBeGreaterThan(0);
+    });
+  });
+
+  it('exports sorted milestone arrays', () => {
+    expect(CONTRIBUTION_MILESTONES).toEqual([1, 10, 100, 250, 500, 1000]);
+
+    expect(STREAK_MILESTONES).toEqual([3, 7, 30, 100]);
+
+    expect([...CONTRIBUTION_MILESTONES].sort((a, b) => a - b)).toEqual(CONTRIBUTION_MILESTONES);
+
+    expect([...STREAK_MILESTONES].sort((a, b) => a - b)).toEqual(STREAK_MILESTONES);
+  });
+
+  it('matches expected constant configuration', () => {
+    expect(SVG_WIDTH).toBe(600);
+    expect(SVG_HEIGHT).toBe(420);
+
+    expect(GHOST_HEIGHT_PX).toBe(4);
+    expect(LOG_SCALE_MULTIPLIER).toBe(12);
+    expect(LINEAR_SCALE_MULTIPLIER).toBe(5);
+
+    expect(MAX_LOG_HEIGHT).toBe(80);
+    expect(MAX_LINEAR_HEIGHT).toBe(50);
+  });
+});