Wire up all GTK UI actions to real functionality

- Settings menu opens PreferencesDialog
- History menu shows HistoryStore entries in a dialog
- Add Files (Ctrl+O) opens FileDialog with image MIME filters
- Process button runs PipelineExecutor in background thread
- Progress bar updates via mpsc channel polled with glib timeout
- Cancel button sets AtomicBool flag to stop processing
- Results page shows real stats (images, sizes, savings, time)
- Open Output Folder launches default file manager
- Process Another Batch resets wizard to step 1
- Toast notifications via ToastOverlay for feedback
- History entries saved after each processing run
- Remove dead_code allows from processing.rs and settings.rs

pixstrip-gtk/src/settings.rs

@@ -1,6 +1,5 @@
 use adw::prelude::*;
 
-#[allow(dead_code)]
 pub fn build_settings_dialog() -> adw::PreferencesDialog {
     let dialog = adw::PreferencesDialog::builder()
         .title("Settings")