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/preset.rs

@@ -67,7 +67,7 @@ impl Preset {
                 crate::operations::CompressConfig::Preset(p) => p.avif_speed(),
                 _ => 6,
             }).unwrap_or(6),
-            output_dpi: 72,
+            output_dpi: 0,
         }
     }