ENG-1596. ENG-1597, ENG-1598: Display, multi-select, and copy extracted nodes (#964)

* ENG-1596: Make extract results panel data-driven

Replaces the skeleton's hardcoded SAMPLE_NODES + fixed EXPANDED_INDICES
with a props-driven MainContent. Accepts ExtractedNode[], derives the
filter tabs from the actual nodes, wires per-row expand/collapse, wires
the type-filter tabs, and renders an empty state when no nodes.

* ENG-1596/1597/1598: Add multi-select and copy-to-clipboard

* Align stub and sample data with ENG-1595 canonical node types

* Fix extract node review comments

* Reset expandedNodes and activeFilter when nodes prop changes

* Scope Select all / counter to active type filter

When a type filter is active, Select all now selects only the filtered
nodes, and the counter and disabled guard track the filtered view.

* Preserve cross-filter selections on Select all

Select all now unions visible indices with existing selection so
selections made under other type filters aren't silently dropped.
Counter expands to show both the in-view count and the global total
whenever hidden selections exist.

Author: sid597