Design the ability to add a new property to an XClass from the Live Data UI

Design the ability to add a new property to an XClass directly from a Live Data that displays the objects of that class. The goal is a design proposal (UX and technical approach) to discuss and validate before any implementation work is filed.

Context

A Live Data that lists the objects of an XClass lets users view, sort, filter and edit values inline. Since XWIKI-17627 it can also create new entries. Adding a new property (a new column) to the class still means leaving the Live Data: open the class editor (or the App Within Minutes wizard), add the property, save, and come back. Users of spreadsheet-like tools (Notion, Airtable, Grist) expect to add a column directly from the table.

Questions the design must answer

• Entry point: where the action lives (a "+" at the end of the header row, the column header menu, the Properties panel), on desktop and on mobile.
• Form: which property types are offered and which settings are asked inline (name, pretty name, type, type-specific settings). Advanced settings can stay in the class editor, through a link.
• Rights: the action is only offered to users who can edit the class document, including its required rights.
• Sources: only some sources can support it (e.g. the liveTable source with a className). A source needs a way to advertise the capability in the Live Data configuration so the UI shows the action only when it applies.
• Live Data configuration: when the macro call lists its properties explicitly, the new property is not displayed. Decide whether the action also updates the macro call (which needs edit right on the page holding the macro) or only shows the column for the current view.
• App Within Minutes: AWM applications own their class, sheet, template and translations. Adding a property from the Live Data must keep them consistent, or be disabled for AWM classes.
• Backend: there is no REST endpoint to add a class property today (XWIKI-12597).

Expected outcome

• UI mockups for the entry point and the form.
• A technical proposal covering the source capability, the configuration update and the backend endpoint.
• A forum proposal, then implementation issues filed from the agreed design.