fix: resolve tag removal bug - tags can now be properly added/removed

[krusty-agent]

Bug Fixed

Issue: "Can't remove tags and they seem to be on every item"

Root Cause

The ItemDetailsModal was receiving stale item snapshots stored in React state. When tags were updated on the server via mutations, the modal still showed the old item.tags data because it was using a snapshot instead of live query data.

Before (broken flow):

1. User opens item modal → setEditingItem(item) stores a snapshot
2. User clicks tag to add/remove → mutation runs on server
3. Server updates item.tags
4. items array from useQuery updates (reactive)
5. BUT: editingItem in React state is still the old snapshot
6. Modal shows stale tags, making it seem like tags can't be removed

Fix

• Changed editingItem and selectedCalendarItem state from Doc<"items"> | null to Id<"items"> | null
• Look up live items from the reactive items array using useMemo
• Modal now receives real-time updates from Convex queries

After (fixed flow):

1. User opens modal → setEditingItemId(item._id) stores only the ID
2. User clicks tag → mutation runs
3. Server updates item.tags
4. items array from useQuery updates
5. editingItem useMemo recalculates with live data
6. Modal shows current tags ✅

Testing

• Build passes ✅
• Tag addition works
• Tag removal works
• Tags correctly show per-item (not shared across items)