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.

Primary fix

Split dndDisabled into dndDisabled (search/patcher/filters/select mode) and reorderDisabled (non-priority sort). DnD stays active for folder drops; only reordering is suppressed. Folder grip handles hidden when reorder is disabled.

Related fixes found during testing

• Grid reorder snap-back — useReorderMods was only sending root mod IDs to the backend, which expects the full set. Fixed by appending folder mods to the reorder call.
• Collision detection — rewrote to use pointerWithin for folder targeting and closestCenter for mod reorder. Prevents false folder highlights and accidental folder drops during reorder.
• Folder drilldown — threaded reorderDisabled through SortableModList so folder views also respect sort mode.
• Folder-mod reorder — added reorderDisabled guard in useUnifiedDnd.handleDragEnd for same-folder reorder.

Happy to split the related fixes into a separate PR if preferred.

Verification

• pnpm check passes (typecheck + lint + format + 268 tests)
• Priority sort: full DnD works in grid and list
• Non-priority sort: folder drops work, reorder blocked
• Folder drilldown respects sort
• Reorder persists when folders contain mods

[Crauzer]

Thanks for the PR, looks good