Duplicate is offered where it can only fail: a full root or slot

Problem/Motivation

Duplicate puts a copy next to the node, in the same root or slot. When that root or slot is full, the copy is refused with "There is no room left here.", from #3626659: Paste, duplicate and preset insert end in a server error when the target is full or gone. The contextual menu still offers Duplicate there, and so does the mod+d shortcut.

It always happens on the root node of a pattern preset: the root of a preset holds one node. It also happens on the root of an entity view override limited by its field, and in a slot whose component sets a maximum.

Nodes inside a slot with room left duplicate as before, in a preset builder too.

The builder already knows. Every limited dropzone carries data-max-items, the root and the slots, from ViewPanelBase::addDropzoneConstraints(). Drag and drop reads it: isDropzoneFull() in dropzone.js refuses a drop into a full dropzone before any request.

Steps to reproduce

1. Open the builder of a pattern preset.
2. Right-click its root node and choose Duplicate.
3. The toast says there is no room left. Nothing told the user before the click.

Proposed resolution

• Disable Duplicate in the contextual menu when the dropzone around the node is full, and say why in the item itself: "Duplicate: no room left here". The menu already rewrites the label of Paste and Remove when it opens, in updateMenuItems().
• The reason is the item's text, not a tooltip. A tooltip needs a hover, which a keyboard user does not have and a disabled item may not react to.
• mod+d keeps sending the request and showing the toast: there is no menu to carry the reason. A disabled state left by an earlier right-click must not swallow it.
• One check shared with drag and drop, not a second copy of isDropzoneFull().
• The server check and its toast stay: the screen can be out of date.
• A Playwright test with a slot that has a maximum: Duplicate is enabled below it, and disabled with its reason once it is reached.

Remaining tasks

• Settle the wording of the reason.
• Decide whether Paste gets the same check on its destination. Paste is already disabled with no reason when nothing was copied: the same rule could cover it.

User interface changes

Duplicate is disabled where the copy has no room, and its label says so.

API changes

None.

Data model changes

None.