Add fourth welcome wizard page that detects installed file managers
(Nautilus, Nemo, Thunar, Dolphin) and offers integration toggles for
each. Follows the design doc specification for the first-run experience.
- Window size/position remembered between sessions via SessionStore
- Step indicator dots now clickable to navigate directly to that step,
with keyboard shortcut hints in tooltips
- File list in add-files dialog shows format and size per image,
header shows total count and total size
- Welcome dialog now saves skill level choice to config
- SessionState extended with window_width, window_height, window_maximized
- Welcome dialog buttons navigate between pages
- Done button closes dialog and marks first_run_complete
- Show welcome dialog on first launch only
- Add first_run_complete field to AppConfig with serde(default)
First-run welcome dialog with skill level and output location setup.
Desktop entry file for GNOME app launcher integration.
Nautilus Python extension with dynamic 'Process with Pixstrip' submenu
that reads both built-in and user presets.
