test: assert ghost city tower invariants (JhaSourav07#820)

## Description

Adds a test to verify ghost city invariants when contribution data
contains only zero values.

The test ensures:

* every computed tower has `isGhost === true`
* every tower height equals `4` (`GHOST_HEIGHT_PX`)

This helps protect the expected ghost city rendering behavior from
future regressions.

Fixes JhaSourav07#728

---

## 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 — 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.
* [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
* [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/svg/layout.test.ts

@@ -103,6 +103,32 @@ describe('computeTowers edge cases', () => {
     expect(towers[0].strokeWidth).toBe(0.5);
   });
 
+  it('marks every tower as a ghost with ghost height for an all-zero calendar', () => {
+    const calendar = {
+      totalContributions: 0,
+      weeks: [
+        {
+          contributionDays: [
+            { contributionCount: 0, date: '2024-06-10' },
+            { contributionCount: 0, date: '2024-06-11' },
+          ],
+        },
+        {
+          contributionDays: [
+            { contributionCount: 0, date: '2024-06-12' },
+            { contributionCount: 0, date: '2024-06-13' },
+          ],
+        },
+      ],
+    } as unknown as ContributionCalendar;
+    const towers = computeTowers(calendar, 'linear', '2024-06-13');
+
+    towers.forEach((tower) => {
+      expect(tower.isGhost).toBe(true);
+      expect(tower.h).toBe(4); // GHOST_HEIGHT_PX
+    });
+  });
+
   it('disables ghost city mode when total visible contributions > 0', () => {
     const calendar = {
       totalContributions: 0,