Wire DPI setting from resize step through to processing job

Add output_dpi field to JobConfig, ProcessingJob, and Preset.
Connect the DPI SpinRow in the resize step's advanced options to
update the config value.

pixstrip-core/src/preset.rs

@@ -44,6 +44,7 @@ impl Preset {
             preserve_directory_structure: false,
             progressive_jpeg: false,
             avif_speed: 6,
+            output_dpi: 72,
         }
     }