Add export depth controls to the Export modal

[Copilot]

Adds advanced export depth controls so users can choose whether to include the selected thought(s) and whether to traverse/export descendants. This makes export behavior match the requested combinations for single- and multi-selection without fetching or trimming unnecessary subtree data afterward.

• Export depth options

  • Adds Export first thought (default on) to skip the selected thought(s) and export their children instead.
  • Adds Export subthoughts (default on) to restrict export to the top-level exported thoughts.
  • Supports the combined case where both are off: export only immediate children, with no deeper descendants.

• Traversal behavior

  • Extends exportContext with depth controls so export shape is determined during traversal rather than by post-processing the rendered output.
  • Preserves existing default behavior for all existing callers outside the modal.

• Preload limiting

  • Extends export preload traversal to stop at the required depth when descendants are excluded.
  • Avoids pulling full subtrees for exports that only need the selected roots or their immediate children.

• Modal behavior

  • Wires the new options into preview text, exported content generation, and export phrase/count calculation.
  • Keeps multi-selection behavior aligned with the filtered selection set already used by the Export modal.

exportContext(state, thoughtId, 'text/plain', {
  includeRoot: false,
  includeDescendants: false,
})
// exports only the selected thought's immediate children

[github-actions]

Vercel preview: https://em-exqabr2x8-cybersemics.vercel.app