Bugfix: correctly handle 'dockutil --remove spacer-tiles' without skipping items or crashing

[vosslab]

Problem

The removeItem("spacer-tiles") branch iterates forward through the array while
removing items. This can cause skipped spacer entries and, depending on the set
of tiles, occasionally lead to an index mismatch error.

Fix

Iterate from the end of the list toward the beginning:

for i in stride(from: items.count - 1, through: 0, by: -1) { ... }

This preserves the original logic while ensuring safe removal of all spacer
tiles.

Testing

• Built with swift build -c release on macOS 26.1
• Added many spacers (spacer, small-spacer, flex-spacer)
• Ran dockutil --remove spacer-tiles
• Verified:
  • No crash
  • All spacers removed
  • Normal items unaffected