fix(checklist): respect drop position when reordering subitems

[reniko]

Summary

Dragging a subitem and dropping it between two siblings always inserted
it at the top of the parent's children list, ignoring the drop position.
Top-level reordering was unaffected.

Steps to reproduce

1. Create a checklist item A with subitems A1, A2, A3.
2. Drag A3 and drop it between A1 and A2.
3. Expected order: A1, A3, A2. Actual order: A3, A1, A2.

Reproduces on develop in Firefox, Edge and Brave.

Root cause

handleDragEnd in app/_hooks/useChecklist.tsx decides whether to drop
before or after the target by matching the drop-zone id prefix
"drop-after::". Top-level drop indicators use that prefix, but subitem
drop indicators use "drop-after-child::" (see NestedChecklistItem.tsx).

"drop-after-child::…".startsWith("drop-after::") is false, so the
check never matched for subitems and position always fell back to
"before" — inserting the dragged item before the target instead of
after it.

The same check is used both for the optimistic local update and for the
position value sent to the reorderItems server action, so the wrong
result was consistent and persisted to disk.

Fix

Match the prefix "drop-after" (without the trailing ::) at both call
sites, so it matches drop-after:: and drop-after-child:: alike. This
mirrors the isDropIndicator check a few lines above, which already uses
the "drop-after" / "drop-before" prefixes. drop-before /
drop-before-child indicators still correctly fall through to "before".

Two-line change, no other files. The server action was already correct —
it just consumes the position it receives.

Tests done

• Subitem A3 dropped between A1 and A2 → A1, A3, A2
• Subitem dropped after the last sibling → lands last
• Subitem dropped before the first sibling → lands first
• Top-level reorder still works (drop before/after top-level items)
• Reload the page — the new order persists