[Feature]: Control the order of many files before converting or merging #927

What would you like Filee to do?

Make it easier to collect many files and control their order before a conversion or a merge, with both the mouse and the keyboard.

Today, when several files are dropped on the Home page, each drop converts or opens the donut right away, so a batch can't be built up over several drops. The merge order window also sorts files by name and lists them as plain text, which makes a long list hard to scan and to reorder.

Why / when would you use it?

When merging many PDFs or pictures, for example scanned pages or a folder of receipts:

• files arrive in several drops, and the first drop already starts a conversion;
• the order is set by name, and fixing it means selecting a row and pressing Alt+Up / Alt+Down one step at a time;
• the file list is text only, so it's hard to tell files apart, and a wrong file can only be removed through a separate step;
• there is no keyboard way to move through the files and act on them.

Ideas for how it could work

• Dropping or picking files on the Home page adds them to a list below the drop zone. A button starts the donut for the list, in the order shown.
• Each file shows a thumbnail (the first page for a PDF, the picture for an image), its place in the order, and an X to remove it.
• Order can be changed by dragging a card, or with the keyboard: left and right arrows move the selection, Alt+Up and Alt+Down move the selected file, and Delete removes it.
• The merge order window uses the same cards and keys.

This overlaps with #860 (reorder and remove before merging) and discussion #669 (thumbnails and order before an images → PDF merge). I'm happy to discuss the open questions here before any pull request:

• Should the list replace the donut when files are dropped, or come in alongside it (for example only when a second file is dropped)?
• Should it live only on the Home page, or also in the Edit PDF pages window, which already has a thumbnail strip with reordering?