Fix 26 bugs, edge cases, and consistency issues from fifth audit pass

Critical: undo toast now trashes only batch output files (not entire dir),
JPEG scanline write errors propagated, selective metadata write result returned.

High: zero-dimension guards in ResizeConfig/fit_within, negative aspect ratio
rejection, FM integration toggle infinite recursion guard, saturating counter
arithmetic in executor.

Medium: PNG compression level passed to oxipng, pct mode updates job_config,
external file loading updates step indicator, CLI undo removes history entries,
watch config write failures reported, fast-copy path reads image dimensions for
rename templates, discovery excludes unprocessable formats (heic/svg/ico/jxl),
CLI warns on invalid algorithm/overwrite values, resolve_collision trailing dot
fix, generation guards on all preview threads to cancel stale results, default
DPI aligned to 0, watermark text width uses char count not byte length.

Low: binary path escaped in Nautilus extension, file dialog filter aligned with
discovery, reset_wizard clears preset_mode and output_dir.

pixstrip-core/src/preset.rs

@@ -5,6 +5,7 @@ use crate::pipeline::ProcessingJob;
 use crate::types::*;
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(default)]
 pub struct Preset {
     pub name: String,
     pub description: String,
@@ -20,6 +21,25 @@ pub struct Preset {
     pub rename: Option<RenameConfig>,
 }
 
+impl Default for Preset {
+    fn default() -> Self {
+        Self {
+            name: String::new(),
+            description: String::new(),
+            icon: "image-x-generic-symbolic".into(),
+            is_custom: true,
+            resize: None,
+            rotation: None,
+            flip: None,
+            convert: None,
+            compress: None,
+            metadata: None,
+            watermark: None,
+            rename: None,
+        }
+    }
+}
+
 impl Preset {
     pub fn to_job(
         &self,
@@ -40,7 +60,7 @@ impl Preset {
             metadata: self.metadata.clone(),
             watermark: self.watermark.clone(),
             rename: self.rename.clone(),
-            overwrite_behavior: crate::operations::OverwriteBehavior::default(),
+            overwrite_behavior: crate::operations::OverwriteAction::default(),
             preserve_directory_structure: false,
             progressive_jpeg: false,
             avif_speed: 6,
@@ -58,6 +78,7 @@ impl Preset {
             Self::builtin_photographer_export(),
             Self::builtin_archive_compress(),
             Self::builtin_fediverse_ready(),
+            Self::builtin_print_ready(),
         ]
     }
 
@@ -119,8 +140,12 @@ impl Preset {
                 suffix: String::new(),
                 counter_start: 1,
                 counter_padding: 3,
+                counter_enabled: true,
+                counter_position: 3,
                 template: None,
                 case_mode: 0,
+                replace_spaces: 0,
+                special_chars: 0,
                 regex_find: String::new(),
                 regex_replace: String::new(),
             }),
@@ -179,8 +204,12 @@ impl Preset {
                 suffix: String::new(),
                 counter_start: 1,
                 counter_padding: 4,
+                counter_enabled: true,
+                counter_position: 3,
                 template: Some("{exif_date}_{name}_{counter:4}".into()),
                 case_mode: 0,
+                replace_spaces: 0,
+                special_chars: 0,
                 regex_find: String::new(),
                 regex_replace: String::new(),
             }),
@@ -204,6 +233,23 @@ impl Preset {
         }
     }
 
+    pub fn builtin_print_ready() -> Preset {
+        Preset {
+            name: "Print Ready".into(),
+            description: "Maximum quality, convert to PNG, keep all metadata".into(),
+            icon: "printer-symbolic".into(),
+            is_custom: false,
+            resize: None,
+            rotation: None,
+            flip: None,
+            convert: Some(ConvertConfig::SingleFormat(ImageFormat::Png)),
+            compress: Some(CompressConfig::Preset(QualityPreset::Maximum)),
+            metadata: Some(MetadataConfig::KeepAll),
+            watermark: None,
+            rename: None,
+        }
+    }
+
     pub fn builtin_fediverse_ready() -> Preset {
         Preset {
             name: "Fediverse Ready".into(),