This commit refactors the WebClient's layout definition to avoid
differences and redundancy between Community & Enterprise editions.
This would ease the maintenance by streamlining both implementation,
keeping them closer. It also allows to have more similar scrolling
handling by default:
- on desktop: `o_content` is the action scroll container (no change)
- on mobile: `o_action` is the action scroll container (down from `html`
node, to keep the navbar)
Part of the SCSS revamp task-2704984
task-2883057
Part-of: odoo/odoo#94134