feat: 📦 Sub-items - Nested checklist items within parent items

[krusty-agent]

Summary

Implements the Sub-items feature that allows creating nested checklist items within a parent item.

Features

• ✅ Sub-item management - Create, check, uncheck, and delete sub-items in ItemDetailsModal
• ✅ Independent checkboxes - Each sub-item has its own checkbox that can be toggled independently
• ✅ Progress tracking - Shows progress bar in modal (e.g., "2/5")
• ✅ ListItem indicator - Shows 📦 badge with progress on parent items (e.g., "📦 2/5")
• ✅ Haptic feedback - Follows existing patterns with success/light/medium/error feedback
• ✅ Permission-aware - Respects canEdit for viewers vs editors
• ✅ Smart sorting - Unchecked items first, checked at bottom

Technical Details

• Backend already had full sub-items support (parentId field, by_parent index, getSubItems query)
• Made SubItems.tsx fully functional (was a placeholder)
• Added useSubItemProgress hook for ListItem to fetch progress
• Uses existing Convex mutations: addItem, checkItem, uncheckItem, removeItem

Screenshots

The sub-items section appears in ItemDetailsModal with:

• Progress bar showing completion
• Individual checkable items
• Add sub-item button (for editors)

Parent ListItem shows:

• 📦 2/5 badge when item has sub-items
• Badge turns green when all sub-items completed