Universal files & directories drop

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• [x] One or multiple files dropped: open them in the reader
• [x] One directory dropped: Prompt user to open it or add it to the bookshelf
• [x] Multiple directories dropped: Add them to the bookshelf

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Done

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Drop a file / files:

https://github.com/user-attachments/assets/ba0480fe-9f14-4b15-9a41-b732e6b7237a

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Drop a directory:

https://github.com/user-attachments/assets/89692c53-9dca-455b-973d-55e682b13d72