This commit improves the user flow for account returns.
When selecting a return, the chatter no longer opens in a side panel.
Instead, a new view is shown displaying:
 - The return card (with stages and buttons),
 - The related checks (as a kanban view),
 - And the chatter on the side.

This unified view offers better clarity and usability. Since the checks
are shown in a kanban view, users can benefit from filters and search
functionality on them.

Additionally, a new stage "Lock" has been added between "Review" and
"Submit" in the VAT return flow. This clarifies to the user that the
return period will be locked before submission. This new stage
generates the closing move and files (xml, pdf, ...) which can then
be used in the submission stage.
For Belgian localization (l10n_be_reports), this change implies that
prorata and potential restitution parameters are now requested at the
"Lock" stage, as they're essential for generating the XML file.

The return checks also see significant changes. They no longer use a
dedicated form view or an associated chatter. The approval process
of a check is now more robust. Both Bookkeepers and Accountants can
validate a check. If a Bookkeeper approves a check, an Accountant
can subsequently "supervise" it. Once supervised by an Accountant,
that specific check can no longer be invalidated by a Bookkeeper.

Finally, Bookkeepers can no longer reset a tax return.

task-4891073