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.
inotify-based folder watcher using the notify crate that detects new
image files, ignores non-image files, and supports start/stop lifecycle.
WatchFolder config struct for preset-linked watched directories.
Preset save/load/list/delete/import/export, config JSON persistence,
session state save/restore, and processing history log with append/clear.
All stored as JSON under ~/.config/pixstrip/.
ImageLoader: load image info (dimensions, format, file size) and pixels.
Discovery: find image files by extension, flat or recursive, single file or directory.
All 9 tests passing.
