fix(library): allow folder drops when non-priority sort is active

[helseaCat]

Fixes #181

Problem

Selecting any sort other than "Priority" disabled all drag-and-drop, including dragging mods into folders. The single dndDisabled flag gated both reordering and folder drops.

Steps to reproduce

1. Create a folder in the library
2. Have a mod outside the folder
3. Switch sort to "Enabled First" (or any non-priority sort)
4. Try to drag the mod into the folder — blocked on v1.8.1

Fix

Split dndDisabled into dndDisabled (search/patcher/filters) and reorderDisabled (non-priority sort). DnD stays active for folder drops; only reordering is suppressed.

Also rewrote collision detection to use pointerWithin for folder targets in all sort modes — folders only highlight when the pointer is directly over them, fixing a pre-existing issue where nearby folders would falsely activate.

Verification

• pnpm check passes (typecheck + lint + format + 268 tests)
• Non-priority sort: folder drops work, reorder blocked, no false animations
• Priority sort: full DnD works, folders require direct hover
• Folder grip handles hidden in non-priority sorts