Add file manager integration install/uninstall logic

Implements actual extension file creation for Nautilus (Python
extension), Nemo (.nemo_action files), Thunar (custom actions XML),
and Dolphin (KDE service menu .desktop). Each extension creates a
"Process with Pixstrip" submenu with all presets listed. Toggle
switches in welcome wizard and settings now call install/uninstall.

pixstrip-core/src/lib.rs

@@ -3,6 +3,7 @@ pub mod discovery;
 pub mod encoder;
 pub mod error;
 pub mod executor;
+pub mod fm_integration;
 pub mod loader;
 pub mod operations;
 pub mod pipeline;