Add all wizard step UIs: workflow, images, resize, convert, compress, metadata, output

Full Adwaita widget-based layouts for all 7 wizard steps with
PreferencesGroups, SwitchRows, SpinRows, ComboRows, FlowBoxes,
ExpanderRows for social media presets, quality slider with named
marks, metadata radio group, and output configuration.

pixstrip-gtk/src/steps/mod.rs

@@ -0,0 +1,7 @@
+pub mod step_compress;
+pub mod step_convert;
+pub mod step_images;
+pub mod step_metadata;
+pub mod step_output;
+pub mod step_resize;
+pub mod step_workflow;