NodeEditor: Display GroupAttribute as collapsible sections instead of nested key/value pairs

[Copilot]

Grouped attributes in the NodeEditor were rendered as nested key/value structures, repeating hierarchy labels and wasting vertical space. This replaces that with collapsible section headers containing child attributes directly.

Description

GroupAttribute entries are now displayed as collapsible section headers (expand/collapse toggle + bold group name) with their children listed below, indented, instead of a flat key → nested-key/value layout. Groups expand by default; users can collapse to reclaim space.

Features list

• GroupAttribute rendered as a collapsible section (chevron toggle + bold label with tooltip) instead of nested key/value pairs
• Left label pane hidden for GroupAttribute rows so the section header spans full row width
• Child AttributeEditor indented (8 px left margin) and collapsed/expanded in sync with the section toggle
• labelWidth propagated from parent instead of hardcoded 100
• showInViewer signal now forwarded from nested AttributeEditor (was missing)
• Fixed attributeDoubleClicked handler to correctly pass both mouse and attr (previously mouse was silently dropped)

Implementation remarks

groupAttributeComponent in AttributeItemDelegate.qml is replaced with a ColumnLayout containing a static header RowLayout and a dynamically-created AttributeEditor (dynamic creation retained to avoid the existing circular-import between AttributeItemDelegate ↔ AttributeEditor). The child editor's visible is bound to groupItem.expanded so the ColumnLayout naturally excludes it from height calculation when collapsed.

The left label Pane gains visible: attribute.type !== "GroupAttribute" so it is excluded from the RowLayout layout, letting the group component fill full width without a redundant label column.