[FIX] base,hr,web: kanban image sizing/look & feel

The image illustrating some kanban cards (like "Employees" or
"Contacts") has a weird scaling and rendering on smaller screen sizes.

Step to reproduce (on small screen):
1. Open Employee
2. Apply a filter
3. Picture is ugly (shape changes to get something pretty weird)

This commit fixes it by:
1. removing the rounding which was only applied (sic) on smaller screens
2. adjusting the size and/or the margins of the image to keep its ratio
   and the alignment with the other kanban cards.

Task ID: 2198419

closes odoo/odoo#45517

Signed-off-by: Pierre Paridans <pparidans@users.noreply.github.com>