Avoid referencing `heatmapGrid.rows` when defining the `Layout.preferredHeight`
and related parameters of the `Grid` components, as `qmllint` in Qt6
reports that the `rows` property is missing from the `Grid` item.
Instead, use the hardcoded constant `7`. Update test assertions in
`scripts/test_ui_regressions.sh` to match.

Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>