[FIX] web: text header appears over the text kanban card

Scrolling inside a grouped kanban with a lot of cards on a column
makes the text header appear over the kanban card due to the missing
background.

Steps to reproduce :
- Open Helpdesk on small screen like a mobile
- Open VIP Support
- Made a Group by "Create date => day" (there should be enough cards on
  a column to scroll)
- Scrolling the column => bug, the text header appears over the kanban
  card

closes odoo/odoo#106330

Signed-off-by: Pierre Paridans (app) <app@odoo.com>