[FIX] mail: dialog properly manage overflow for long content

Before this commit, when content of dialog was long, content
overflow but was not scrollable, which makes bottom unreachable,
which is especially a problem when buttons are at the bottom.

This problem solves the issue by adding classname `modal` to
dialog. This classname puts proper handling of overflow of
dialog and its content.

Also remove `align-items-center` so that top doesn't overflow
in isolation from the rest of content, which made top unreachable.

Task-2973866

closes odoo/odoo#99620

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>