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/pipeline.rs

@@ -25,6 +25,7 @@ pub struct ProcessingJob {
     pub preserve_directory_structure: bool,
     pub progressive_jpeg: bool,
     pub avif_speed: u8,
+    pub output_dpi: u32,
 }
 
 impl ProcessingJob {
@@ -47,6 +48,7 @@ impl ProcessingJob {
             preserve_directory_structure: false,
             progressive_jpeg: false,
             avif_speed: 6,
+            output_dpi: 72,
         }
     }
 

pixstrip-core/src/preset.rs

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