[FIX] website: properly handle "100% height" snippet when in popups

Before this commit, when a "100% height" snippet was set in a popup
snippet, it was still computed to take full-screen height minus the size
of the navbar.

Now, it will properly be full-height.

closes odoo/odoo#76897

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>