Add assertion for non-ghost towers in active calendar (JhaSourav07#958)

## Description
Adds an assertion to verify that all towers have `isGhost === false`
when the calendar contains at least one contribution.

closes JhaSourav07#729 

## Pillar

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

## Visual Preview
N/A

## 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`).
- [ ] 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.
- [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: parneetbrar234-svg

lib/svg/layout.test.ts

@@ -142,6 +142,7 @@ describe('computeTowers edge cases', () => {
       ],
     } as unknown as ContributionCalendar;
     const towers = computeTowers(calendar, 'linear', '2024-06-10');
+    expect(towers.every((tower) => tower.isGhost === false)).toBe(true);
     expect(towers[0].isGhost).toBe(false);
     expect(towers[0].h).toBe(0); // 0 count non-ghost = 0 height
     expect(towers[0].strokeOpacity).toBe(0);