Enable preview display of selected files

[Slowhand0309]

This pull request adds a file preview feature to the QuickShelf app, allowing users to preview non-directory files in a floating side panel when they are selected. The implementation introduces a new SlidePanelPreview class for managing the preview panel and updates the UI to include a preview button for eligible items.

Issue: #49

Preview feature integration:

• Added the SlidePanelPreview singleton class in QuickShelf/Views/Preview/SidePanelPreview.swift to display file previews in a floating panel beside the main window, including ESC key handling to close the panel.
• Updated ContentView.swift to pass an onPreview callback to ShelfItemView, which triggers the preview panel for selected files, and ensures the panel is hidden when the view disappears. [1] [2]

UI enhancements for file selection and preview:

• Modified ShelfItemView to accept isSelected and onPreview parameters, displaying an "eye" button for previewing non-directory items when selected, with support for spacebar keyboard shortcut. [1] [2]

Framework imports for preview functionality:

• Imported QuickLook in ContentView.swift to enable file preview capabilities.