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-core/src/fm_integration.rs

@@ -151,8 +151,8 @@ fn install_nautilus() -> Result<()> {
              \x20       item.connect('activate', self._on_preset, '{}', files)\n\
              \x20       submenu.append_item(item)\n\n",
             name.replace(' ', "_"),
-            name.replace('\'', "\\'"),
-            name.replace('\'', "\\'"),
+            name.replace('\\', "\\\\").replace('\'', "\\'"),
+            name.replace('\\', "\\\\").replace('\'', "\\'"),
         ));
     }