Add window persistence, clickable step indicator, file list improvements

- Window size/position remembered between sessions via SessionStore
- Step indicator dots now clickable to navigate directly to that step,
  with keyboard shortcut hints in tooltips
- File list in add-files dialog shows format and size per image,
  header shows total count and total size
- Welcome dialog now saves skill level choice to config
- SessionState extended with window_width, window_height, window_maximized

pixstrip-core/src/storage.rs

@@ -172,6 +172,9 @@ pub struct SessionState {
     pub last_output_dir: Option<String>,
     pub last_preset_name: Option<String>,
     pub current_step: u32,
+    pub window_width: Option<i32>,
+    pub window_height: Option<i32>,
+    pub window_maximized: bool,
 }
 
 pub struct SessionStore {