feat(tree-view-table-layout): shift-click a chevron to expand/collapse the whole subtree

[seltzdesign]

Problem

The expand/collapse chevron only ever toggles one level. Fully opening or closing a deep branch means clicking down (or up) through every intermediate node by hand, which is tedious on large trees.

Fix

Add a power-user gesture: shift-click a chevron to expand or collapse the entire subtree under that node in one action. A plain click is unchanged (single level).

Implementation:

• src/components/table-row.vue — the chevron's click handler now forwards the modifier state: $emit('toggle-children', $event.shiftKey). A normal click emits false, a shift-click emits true.

• src/components/v-table.vue — onToggleChildren(item, deep = false) gains a deep flag. When deep is true it delegates to a new toggleChildrenDeep(item), which:

  • picks a single target collapsed state (the opposite of the clicked node's current state), so the whole branch ends up consistently open or consistently closed regardless of the mixed states it started in;
  • collects the clicked node plus every descendant container (descendants that themselves have children) from internalItems;
  • drives only the containers that are not already at the target state to that state, reusing the existing toggleItem / collapseChildren primitives.

  The template binding is updated to pass the flag through: @toggle-children="(deep) => onToggleChildren(item, deep)".

Defaulting deep to false keeps the single-level behaviour for every existing caller and for plain clicks.

Verification

• Shift-clicking a collapsed top-level node expands the full branch (all nested containers open) in one gesture; shift-clicking it again collapses the whole branch.
• Shift-clicking a branch that is in a mixed state (some children open, some closed) drives the entire branch to a single consistent state rather than toggling each node independently.
• Plain (non-shift) clicks still toggle exactly one level, unchanged.

Note for reviewers

This PR touches src/components/table-row.vue, but only the @click emit hunk in the <template> (forwarding $event.shiftKey). A separate PR ("fix: point the collapsed-row chevron right") also touches this file, but in a different, non-overlapping hunk — the transform: rotate(...) rule in the <style> block. The two changes are independent and do not conflict.