[jaygeorge]

Depends on #15676.

Description of the Problem

You can drag assets deeper into subfolders, but there's no way to drag one back up. To move an asset into a parent folder, or back to the top, you have to use the Move action instead.

Moving something also didn't feel great. The item showed a spinner, waited on the server, then vanished when the listing refreshed. You could only drag one asset at a time.

What this PR Does

The breadcrumbs above the asset browser are now drop targets, so you can drag an asset or folder onto any parent folder, including "All".

• The breadcrumb you're over gets the same blue ring as a folder in grid view. In list view, folders now get the ring around their icon and name too, instead of the faint row background.
• Only items you're allowed to move light up a drop target. A folder can't be dropped onto itself, and anything already in the destination is skipped.
• While searching, results can come from anywhere, so the current folder's breadcrumb accepts drops too.

Moving multiple assets

Dragging a selected asset now brings the rest of the selection with it. Dragging an unselected asset still moves just that one. Anything in the selection you can't move stays behind.

Drop animation

Dropping onto a folder plays a small macOS-style animation. The folder icon opens, the file drops in, and it snaps shut. When you move several files, it shows a fanned bundle of up to three thumbnails. In list view the same animation plays on the smaller folder icon, and a breadcrumb shows the open-folder icon while its folder is receiving files. The animation respects prefers-reduced-motion.

Optimistic UI

The UI responds straight away instead of waiting on the server:

1. On drop, the items dim and the request starts immediately. The progress bar in the header runs as usual.
2. After a short delay, the folder opens and the items fade out. Once they've faded, the remaining items slide into place.
3. The folder stays open until the server finishes, so large moves don't look done before they are. Fast moves still keep it open long enough to see the file land.
4. A "Moved to …" or "Moved 3 items to …" toast appears, the folder closes, and the listing refreshes.

If the move fails, the items come back, the folder closes, and an error toast explains why. In search results, moved items stay in the listing, dimmed, because the results span every folder. Concurrent drops are tracked separately, so dropping into two folders, or twice into the same one, animates correctly.

The move itself lives in Browser rather than in the grid and list components, so it carries on correctly if you switch views mid-move. Each move is settled when the refreshed listing comes back, and the selection is updated in that same render. Moved assets get new IDs, so the action's own replaceInSelections callback keeps them selected rather than them silently dropping out (which matters in the asset picker, where the selection is what you're picking).

Tests cover which items a drag carries and the move flow's timing: when the folder opens and closes, failures, and concurrent drops.

drag-to-breadcrumbs.mp4

How to Reproduce

1. Go to Assets and open a folder at least two levels deep.
2. Drag an asset onto "All" or a parent folder in the breadcrumbs. The folder icon animates and the asset moves there.
3. Select a few assets and drag one of them onto a folder. They all move, and the folder shows a bundle of thumbnails.