Added image preview

• It's able to preview image files ending with .png,, .jpg, .jpeg, .gif, .bmp, .webp now.
• Optimized merging/copying/moving preparation by calculating source size and reclaimable overwrite space in a single traversal.
• Reworked permission checks to fix the problem that it was not able to paste/move things into the root of /mnt/ext?/ and /mnt/usb?/.
• Added completion dialog for long copy and move tasks, including duration, transferred size, and file count.
• Improved large-directory rendering, scrolling performance, and loading feedback.