feat: add nested hierarchical groups for connection list

[datlechin]

Summary

Closes #478

• Nested groups — groups can contain subgroups up to 3 levels deep, following the existing FavoriteTreeItem + FavoritesTabView pattern
• Tree data model — new ConnectionGroupTreeNode enum with recursive buildGroupTree, filterGroupTree, flattenVisibleConnections, plus utility functions for cascade delete, circular reference prevention, and depth validation
• Native macOS UX — replaced flat Section headers with recursive DisclosureGroup for expand/collapse, matching the favorites sidebar pattern
• Cascade delete — deleting a parent group removes all descendants and ungroups their connections
• Hierarchical group picker — "Move to Group" menu and CreateGroupSheet now show nested hierarchy with parent selection
• iCloud sync — parentId and sortOrder fields added to CKRecord mapping
• 30 unit tests for tree building, filtering, flattening, cycle detection, depth cap, and cascade logic

Test plan

• Create top-level group, subgroup, and 3rd-level subgroup
• Verify "New Subgroup" disabled at depth 3
• Expand/collapse groups via disclosure triangle and Ctrl+H/L
• Delete parent group — verify cascade removes subgroups, ungroups connections
• Rename group — verify sibling duplicate check
• Search filters connections while preserving parent group chain
• Drag to reorder connections within a group and at top level
• "Move to Group" context menu shows hierarchy
• Group picker in connection form shows nested hierarchy
• Keyboard navigation (arrows, Ctrl+J/K, Return, Cmd+A, Escape)
• Double-click to connect
• Multi-select context menu (batch connect, export, delete)
• Run unit tests: xcodebuild test -only-testing:TableProTests/ConnectionGroupTreeTests