Wire resize algorithm selection, overwrite behavior, and fix rotation/flip scope

- Add ResizeAlgorithm enum (Lanczos3/CatmullRom/Bilinear/Nearest) to core
- Thread algorithm selection from UI ComboRow through ProcessingJob to resize_image
- Add OverwriteBehavior enum (AutoRename/Overwrite/Skip) to core
- Implement overwrite handling in executor with auto-rename suffix logic
- Wire overwrite behavior from output step through to processing job
- Fix rotation/flip to apply when resize step is enabled, not just adjustments

pixstrip-gtk/src/steps/step_resize.rs

@@ -453,6 +453,12 @@ pub fn build_resize_page(state: &AppState) -> adw::NavigationPage {
             jc.borrow_mut().flip = row.selected();
         });
     }
+    {
+        let jc = state.job_config.clone();
+        algorithm_row.connect_selected_notify(move |row| {
+            jc.borrow_mut().resize_algorithm = row.selected();
+        });
+    }
 
     scrolled.set_child(Some(&content));