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-gtk/src/app.rs

@@ -19,6 +19,7 @@ pub struct JobConfig {
     pub resize_height: u32,
     pub allow_upscale: bool,
     pub resize_algorithm: u32,
+    pub output_dpi: u32,
     // Adjustments
     pub adjustments_enabled: bool,
     pub rotation: u32,
@@ -226,6 +227,7 @@ fn build_ui(app: &adw::Application) {
             resize_height: if remember { sess_state.resize_height.unwrap_or(0) } else { 0 },
             allow_upscale: false,
             resize_algorithm: 0,
+            output_dpi: 72,
             adjustments_enabled: false,
             rotation: 0,
             flip: 0,
@@ -1454,6 +1456,7 @@ fn run_processing(_window: &adw::ApplicationWindow, ui: &WizardUi) {
     }
 
     job.preserve_directory_structure = cfg.preserve_dir_structure;
+    job.output_dpi = cfg.output_dpi;
     job.overwrite_behavior = match cfg.overwrite_behavior {
         1 => pixstrip_core::operations::OverwriteBehavior::AutoRename,
         2 => pixstrip_core::operations::OverwriteBehavior::Overwrite,