test(layout): verify all towers have non-negative height values (JhaSourav07#2222)

## Description

Adds a regression test to ensure all towers returned by computeTowers
have non-negative height values.

## Changes
Added a test covering varied contribution counts (0, 1, 5, 20).
Verifies every generated tower satisfies h >= 0.

Fixes JhaSourav07#727 

## Pillar


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

## Visual Preview

## 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/layout.test.ts

@@ -215,6 +215,27 @@ describe('computeTowers edge cases', () => {
     // Assert intensityLevel is calculated correctly based on lines of code (60/60 = 100%, so intensity 4)
     expect(testTower.intensityLevel).toBe(4);
   });
+  it('ensures all tower heights are non-negative', () => {
+    const calendar = {
+      totalContributions: 26,
+      weeks: [
+        {
+          contributionDays: [
+            { contributionCount: 0, date: '2024-06-10' },
+            { contributionCount: 1, date: '2024-06-11' },
+            { contributionCount: 5, date: '2024-06-12' },
+            { contributionCount: 20, date: '2024-06-13' },
+          ],
+        },
+      ],
+    } as unknown as ContributionCalendar;
+
+    const towers = computeTowers(calendar, 'linear', '2024-06-13');
+
+    towers.forEach((tower) => {
+      expect(tower.h).toBeGreaterThanOrEqual(0);
+    });
+  });
 });
 
 it('assigns correct row and col values based on week/day position', () => {