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-core/src/preset.rs

@@ -31,6 +31,7 @@ impl Preset {
             output_dir: output_dir.into(),
             sources: Vec::new(),
             resize: self.resize.clone(),
+            resize_algorithm: crate::operations::ResizeAlgorithm::default(),
             rotation: self.rotation,
             flip: self.flip,
             adjustments: None,
@@ -39,6 +40,7 @@ impl Preset {
             metadata: self.metadata.clone(),
             watermark: self.watermark.clone(),
             rename: self.rename.clone(),
+            overwrite_behavior: crate::operations::OverwriteBehavior::default(),
             preserve_directory_structure: false,
             progressive_jpeg: false,
             avif_speed: 6,