Fix drag-and-drop for sandboxed Flatpak apps (e.g., Telegram)

[RoNiN-shadow]

Fixes #53

Description

This PR fixes an issue where dragging files into strictly sandboxed Flatpak applications (like Telegram) resulted in "File is empty" or "Permission denied" errors.

Changes Made:

• Switched to native FileList: Replaced ContentProvider::for_bytes with ContentProvider::for_value(&FileList). This ensures that GTK correctly registers the dragged files with xdg-desktop-portal, granting the target Flatpak app the necessary read permissions.
• Fixed URI parsing: File::for_uri in generate_content_provider. The function was receiving already-formatted URIs (like file:///...) from setup_drag_source_all.
• Bumped GTK feature: Bumped the gtk4 feature flag in Cargo.toml from v4_6 to v4_8 to utilize the FileList::from_array method.

Tested locally on Arch Linux with Flatpak Telegram Desktop. Works flawlessly. Let me know if the v4_8 version bump is acceptable for the project's compatibility goals!

[nik012003]

Thank you for the PR, the changes look good!
v4.8 has been out for a few years now (it's the latest version in debian bookworm) so I'm perfectly fine with bumping it.
I will pull, do some testing and release it when I get back home.

[nik012003]

Merged and released in v0.4.12
Thanks for the good work!