Linux Cinnamon Desktop Detection for File Dialogs

Problem (GitHub #116)

On Linux Mint Cinnamon, XDG_CURRENT_DESKTOP is set to X-Cinnamon, which lowercases to x-cinnamon. The desktop detection code only matched "cinnamon", causing Cinnamon to be misclassified as a non-native desktop that requires xdg-desktop-portal. This led to:

1. Dialog cancellation (user pressing Cancel) being misreported as a portal failure.
2. Portal installation instructions recommending xdg-desktop-portal-wlr (wrong for Cinnamon).

Solution

Three fixes in src/files/dialogs.rs (and related call sites):

1. Native Desktop Detection

Added "x-cinnamon" to the has_native match list in detect_linux_desktop() so that XDG_CURRENT_DESKTOP=X-Cinnamon is correctly recognized as a desktop with native file dialog support.

2. Portal Install Instructions

Updated portal_install_instructions() to accept the detected desktop environment and return Cinnamon-appropriate packages (xdg-desktop-portal-xapp and xdg-desktop-portal-gtk) instead of the wlroots-specific xdg-desktop-portal-wlr.

3. Cancellation Handling

Changed all dialog functions (open_folder_dialog, open_multiple_files_dialog, save_file_dialog) and the export dialog in app/export.rs to always return DialogResult::Cancelled when rfd returns None. Previously, None on portal-requiring desktops was classified as DialogResult::Failed, which showed an intrusive error dialog even for normal user cancellation.

Debug-level logs are still emitted on portal-requiring desktops so the issue is visible in logs without false-positive error dialogs.

Files Changed

File	Change
src/files/dialogs.rs	Added x-cinnamon to native list, desktop-aware portal instructions, cancellation fix
src/app/file_ops.rs	Updated portal_install_instructions() call to pass desktop env
src/app/export.rs	Applied same cancellation fix to export save dialog