(UX) File Gallery: Drag-and-Drop Reordering

[n2ojim]

Scope & Context

In industries where the sequence of images matters (like determining the order of a property slideshow), users need a visual way to reorder files.
(Note: This PR relies on the backend capability to store file positions, as described in feature request #21726 ).

Current behavior

There is no way to visually drag and drop thumbnails to change their order in the sequence. The order is static, usually based on upload time.

Expected behavior

1. Position Indicator: Show the numerical position of the file right before its name in the gallery view (e.g., "1. LivingRoom.jpg", "2. Kitchen.jpg").
2. Drag-and-Drop Reordering: Users should be able to click, hold, and drag thumbnails within the grid to physically change their order in the sequence. The UI should provide visual feedback (like a ghost image or a drop indicator line) during the drag.

Technical inputs

• Integrate @dnd-kit/core and @dnd-kit/sortable (which are already used in the Twenty codebase) around the FileGallery grid component to handle the visual reordering of the thumbnails.
• Implement an optimistic UI update to instantly reflect the new order locally onDragEnd.
• Trigger the updateAttachmentPositions (or the equivalent mutation defined in the backend PR) passing the new array of ordered IDs to persist the sequence in the database.