feat(ui): group nodes by category in add-node dialog

[Pfannkuchensack]

Summary

Add collapsible category grouping to the node picker command palette. Categories are parsed from the backend schema and displayed as expandable sections with caret icons. All categories auto-expand when searching.

Related Issues / Discussions

#7429

QA Instructions

1. Open the workflow editor
2. Open the add-node dialog (right-click or shortcut)
3. Verify nodes are displayed as a flat list by default
4. Open workflow editor settings (gear icon) and enable "Group Nodes by Category"
5. Reopen the add-node dialog and verify nodes are now grouped by category with collapsible headers
6. Click category headers to expand/collapse
7. Type a search term and verify all matching categories auto-expand, with exact title matches shown first
8. Clear search and verify categories return to collapsed state
9. Disable the setting again and verify it returns to a flat list

Merge Plan

No special merge considerations.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)