Description of the issue/feature this PR addresses:

The "Computed Arch" debug dialog displays the fully computed view architecture, after inheritance, xpaths and studio customizations have been applied, in a read-only pre block. Extracting that text previously required manually selecting it in the browser, which is unreliable for long or deeply indented XML.

Being able to copy this text to the clipboard helps with several common scenarios:

- Debugging inherited views: when a view looks wrong and it is not clear which of several inheriting modules caused it, the computed  arch can be diffed against the base view's arch to see exactly what  changed.
- Writing new xpath inherit rules: the exact final structure (attribute values, node ordering) is needed to write a correct
  expr="//field[@name='x']", which is easier to do in a scratch file or  editor with search than by eyeballing a read-only dialog.
- Bug reports and support tickets: the computed arch can be pasted into a Slack message, GitHub issue or support ticket so others can  inspect the view without needing access to the database.
- Studio customizations: consultants often need the underlying  computed arch to understand why a Studio-added field appears in an unexpected position.
- Comparing environments: the arch from a customer's staging instance can be diffed against production to spot config drift, e.g. a view customization made directly on prod.


Current behavior before PR:

In the "Computed Arch" debug dialog, the only way to get the arch text out of the browser is to manually click-drag to select it inside the pre block, then copy it. This is error-prone for long or deeply indented XML, where whitespace and line breaks are easy to select incorrectly or truncate.


Desired behavior after PR is merged:

A "Copy Arch" button is added next to "Close" in the dialog's footer. Clicking it copies the full computed arch to the clipboard in one click, with a brief "Copied" confirmation, removing the need to manually select the text.

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Forward-Port-Of: odoo/odoo#286797
Forward-Port-Of: odoo/odoo#286619