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/tests/operations_tests.rs

@@ -91,8 +91,12 @@ fn rename_config_simple_template() {
         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(),
     };
@@ -107,11 +111,101 @@ fn rename_config_with_suffix() {
         suffix: "_web".into(),
         counter_start: 1,
         counter_padding: 2,
+        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(),
     };
     let result = config.apply_simple("photo", "webp", 5);
     assert_eq!(result, "photo_web_05.webp");
 }
+
+#[test]
+fn rename_counter_overflow_saturates() {
+    let config = RenameConfig {
+        prefix: String::new(),
+        suffix: String::new(),
+        counter_start: u32::MAX,
+        counter_padding: 1,
+        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(),
+    };
+    // Should not panic - saturating arithmetic
+    let result = config.apply_simple("photo", "jpg", u32::MAX);
+    assert!(result.contains("photo"));
+}
+
+#[test]
+fn metadata_config_custom_selective() {
+    let config = MetadataConfig::Custom {
+        strip_gps: true,
+        strip_camera: false,
+        strip_software: true,
+        strip_timestamps: false,
+        strip_copyright: false,
+    };
+    assert!(config.should_strip_gps());
+    assert!(!config.should_strip_camera());
+    assert!(!config.should_strip_copyright());
+}
+
+#[test]
+fn metadata_config_custom_all_off() {
+    let config = MetadataConfig::Custom {
+        strip_gps: false,
+        strip_camera: false,
+        strip_software: false,
+        strip_timestamps: false,
+        strip_copyright: false,
+    };
+    assert!(!config.should_strip_gps());
+    assert!(!config.should_strip_camera());
+    assert!(!config.should_strip_copyright());
+}
+
+#[test]
+fn metadata_config_custom_all_on() {
+    let config = MetadataConfig::Custom {
+        strip_gps: true,
+        strip_camera: true,
+        strip_software: true,
+        strip_timestamps: true,
+        strip_copyright: true,
+    };
+    assert!(config.should_strip_gps());
+    assert!(config.should_strip_camera());
+    assert!(config.should_strip_copyright());
+}
+
+#[test]
+fn watermark_rotation_variants_exist() {
+    let rotations = [
+        WatermarkRotation::Degrees45,
+        WatermarkRotation::DegreesNeg45,
+        WatermarkRotation::Degrees90,
+        WatermarkRotation::Custom(30.0),
+    ];
+    assert_eq!(rotations.len(), 4);
+}
+
+#[test]
+fn rotation_auto_orient_variant() {
+    let rotation = Rotation::AutoOrient;
+    assert!(matches!(rotation, Rotation::AutoOrient));
+}
+
+#[test]
+fn overwrite_action_default_is_auto_rename() {
+    let default = OverwriteAction::default();
+    assert!(matches!(default, OverwriteAction::AutoRename));
+}