Fix 40+ bugs from audit passes 9-12

- PNG chunk parsing overflow protection with checked arithmetic
- Font directory traversal bounded with global result limit
- find_unique_path TOCTOU race fixed with create_new + marker byte
- Watch mode "processed" dir exclusion narrowed to prevent false skips
- Metadata copy now checks format support before little_exif calls
- Clipboard temp files cleaned up on app exit
- Atomic writes for file manager integration scripts
- BMP format support added to encoder and convert step
- Regex DoS protection with DFA size limit
- Watermark NaN/negative scale guard
- Selective EXIF stripping for privacy/custom metadata modes
- CLI watch mode: file stability checks, per-file history saves
- High contrast toggle preserves and restores original theme
- Image list deduplication uses O(1) HashSet lookups
- Saturation/trim/padding overflow guards in adjustments

pixstrip-gtk/src/steps/step_watermark.rs

@@ -210,6 +210,10 @@ pub fn build_watermark_page(state: &AppState) -> adw::NavigationPage {
             .height_request(48)
             .build();
 
+        btn.update_property(&[
+            gtk::accessible::Property::Label(&format!("Watermark position: {}", name)),
+        ]);
+
         let icon = if i == cfg.watermark_position as usize {
             "radio-checked-symbolic"
         } else {