Improve Images, Compress, Output, Workflow steps

- Images step: folder drag-and-drop with recursive image scanning, per-file
  list with format and size info, total file size in header, supported
  formats label in empty state
- Compress step: per-format quality controls moved into AdwExpanderRow,
  improved quality level descriptions
- Output step: dynamic image count with total size from loaded_files,
  initial overwrite behavior from config
- Workflow step: properly handle MetadataConfig::Custom in preset import,
  mapping all custom metadata fields to JobConfig

pixstrip-gtk/src/steps/step_workflow.rs

@@ -213,9 +213,14 @@ fn apply_preset_to_config(cfg: &mut JobConfig, preset: &Preset) {
             cfg.metadata_enabled = true;
             cfg.metadata_mode = MetadataMode::KeepAll;
         }
-        Some(MetadataConfig::Custom { .. }) => {
+        Some(MetadataConfig::Custom { strip_gps, strip_camera, strip_software, strip_timestamps, strip_copyright }) => {
             cfg.metadata_enabled = true;
-            cfg.metadata_mode = MetadataMode::StripAll;
+            cfg.metadata_mode = MetadataMode::Custom;
+            cfg.strip_gps = *strip_gps;
+            cfg.strip_camera = *strip_camera;
+            cfg.strip_software = *strip_software;
+            cfg.strip_timestamps = *strip_timestamps;
+            cfg.strip_copyright = *strip_copyright;
         }
         None => {
             cfg.metadata_enabled = false;