Fix 12 medium-severity bugs across all crates

- Escape backslashes in Nautilus preset names preventing Python injection
- Fix tiled watermarks starting at (spacing,spacing) instead of (0,0)
- Fix text watermark width overestimation (1.0x to 0.6x multiplier)
- Fix output_dpi forcing re-encoding for metadata-only presets
- Fix AVIF/WebP compression detection comparing against wrong preset values
- Add shared batch_updating guard for Ctrl+A/Ctrl+Shift+A select actions
- Fix overwrite conflict check ignoring preserve_directory_structure
- Add changes_filename()/changes_extension() for smarter overwrite checks
- Fix watch folder hardcoding "Blog Photos" preset
- Fix undo dropping history for partially-trashed batches
- Fix skipped files inflating size statistics
- Make CLI watch config writes atomic

pixstrip-gtk/src/steps/step_images.rs

@@ -550,8 +550,8 @@ fn build_loaded_state(state: &AppState) -> gtk::Box {
         .spacing(0)
         .build();
 
-    // Guard flag to prevent O(n^2) update cascade during batch checkbox operations
-    let batch_updating: Rc<std::cell::Cell<bool>> = Rc::new(std::cell::Cell::new(false));
+    // Use the shared batch_updating flag from AppState
+    let batch_updating = state.batch_updating.clone();
 
     // Toolbar
     let toolbar = gtk::Box::builder()