feat: drag-and-drop column reordering and always-visible sort icons - #1877

This change is 

What this does

Two quality-of-life improvements to the collection view table headers:

1. Columns can be reordered by drag-and-drop

• a drag handle appears on the left of each column header when hovering it
• dragging it over another column moves the whole column (header + cells) there, with a visual drop indicator
• the order is persisted per database/collection in the browser's localStorage (key mongo-express:columnOrder:<db>/<collection>) and is re-applied on every page load — it survives searches, sorting and pagination
• columns not present in the saved order (new fields) keep their natural position at the end
• purely client-side: no server or API change

The drag only starts from the dedicated handle, on purpose: making the whole <th> draggable would swallow the click event on any slight mouse movement and break click-to-sort.

2. Clearer sort affordance

• every column now always shows a sort icon: dimmed when the column is not sorted, highlighted up/down arrow when it is — previously nothing indicated that headers are clickable until a column was sorted
• headers no longer wrap on several lines when a column name is short and the sort icon used to overflow (white-space: nowrap)
• header text is not selectable anymore (user-select: none), which previously interfered with click-to-sort (a click that selected text was ignored)

Before, arrow was on 2 lines

After, arrow is always on same line

How to verify

1. Open any collection
2. Hover a column header → a drag handle appears; drag it onto another column → the column moves
3. Reload the page, navigate, search, sort → the custom column order is kept
4. Open the same collection in another browser/profile → default order (storage is local)
5. Click headers → sorting still works exactly as before