Add clear GPS endpoint + frontend button

- DELETE /api/assets/{id}/gps sets lat/lng to NULL
- Detail view shows 'Clear GPS' button when asset has GPS coords
- Confirmation modal before clearing
- Tests: 204 success + 404 not found

tests/test_server.py

@@ -432,6 +432,31 @@ class TestDeleteAsset:
         assert response.status_code == 404
 
 
+class TestClearAssetGps:
+    """Clear GPS endpoint."""
+
+    def test_clear_gps_returns_204(self, client):
+        """Clear GPS returns 204 and sets lat/lng to null."""
+        r = client.post("/api/assets", json={
+            "machine_id": "GPS001", "name": "Has GPS",
+            "latitude": 28.5, "longitude": -81.3
+        })
+        asset_id = r.json()["id"]
+        response = client.delete(f"/api/assets/{asset_id}/gps")
+        assert response.status_code == 204
+
+        # Verify lat/lng are cleared
+        r2 = client.get(f"/api/assets/{asset_id}")
+        assert r2.status_code == 200
+        data = r2.json()
+        assert data["latitude"] is None
+        assert data["longitude"] is None
+
+    def test_clear_gps_not_found_returns_404(self, client):
+        response = client.delete("/api/assets/99999/gps")
+        assert response.status_code == 404
+
+
 # ─── Task 7: GET /api/assets/search?machine_id= ───────────────────────────