Add a selection mode/mechanism to the main Index

Current UX for quickly working with a lot of archives is painful in a few ways, so this is a design draft to try and fix that.
I'd already briefly mentioned it here: https://github.com/Difegue/LANraragi/issues/570#issuecomment-1550023455 , but it's more generic than just batch operations and doesn't require a heavy frontend refactor:

• Current Index page should have a new toggle to enable "Multi-Select Mode"
  • (or maybe make it one of the carousel dropdown options? I'm worried about discoverability if doing it that way tho)
• When this toggle is enabled, the topmost carousel is emptied out, and checkboxes appear next to each archive.
• Search works as normal -- Ticking a checkbox adds it to the carousel for quick review of your selected archives, no matter what you're searching for.
• Once at least one file is selected, you can choose to run Batch Operations on the selection.
  • (which will probably be a popup with an iframe to the current batch page or something because that's entirely too much complexity to move into the index)
  • Some simpler options could eventually live here alongside Batch, like quick deletion, creating/adding to a category(or meta-archive when those come!), or IceBreeze's crazy poweruser batch syntax mentioned here

Just saw this, so I stopped to add a few random thoughts to the discussion:

• for me, using check-boxes is convenient for working with few archives (one page?), but as the number grows, it becomes tedious to select the items one by one
• in checkbox mode it would be convenient if clicking everywhere on a tile would select/deselect the item (it would avoid the risk of opening the link and losing the entire selection)
• I think that the version with the command bar is better for working with many archives
• you can see and navigate the search pages before applying the command without having to make too many GUI changes.
• check boxes allow you to select without having to go crazy with the refinement of the search filter
• you could also use the search bar as a command bar by choosing a prefix to indicate that you want to insert a command
  • the user must understand that the command will be applied to the last search
  • or you need to think of a syntax to express the command together with the filter
• it would be nice to extend the plugin system so that you can add more commands

I think it'd be doable to allow for selecting a range of archives in checkbox mode using the shift key.

This South-plus post has some UX for checkbox selection. (And tankoubons as well)

I'm not sure if such a feature would be easy to implement, so please consider this as a hypothetical idea: If it were possible to filter files by their names or tags on a filter page, group those specific files into a certain category, and then perform batch operations only within that category, I imagine this could address many of the issues people encounter when managing files.