Add AppConfig with overwrite behavior, skill level, thread count settings

All 3 config tests passing.

pixstrip-core/tests/config_tests.rs

@@ -0,0 +1,27 @@
+use pixstrip_core::config::*;
+
+#[test]
+fn default_config() {
+    let config = AppConfig::default();
+    assert_eq!(config.output_subfolder, "processed");
+    assert_eq!(config.overwrite_behavior, OverwriteBehavior::Ask);
+    assert!(config.remember_settings);
+    assert_eq!(config.skill_level, SkillLevel::Simple);
+    assert_eq!(config.thread_count, ThreadCount::Auto);
+    assert_eq!(config.error_behavior, ErrorBehavior::SkipAndContinue);
+}
+
+#[test]
+fn config_serialization_roundtrip() {
+    let config = AppConfig::default();
+    let json = serde_json::to_string(&config).unwrap();
+    let deserialized: AppConfig = serde_json::from_str(&json).unwrap();
+    assert_eq!(deserialized.output_subfolder, config.output_subfolder);
+    assert_eq!(deserialized.overwrite_behavior, config.overwrite_behavior);
+}
+
+#[test]
+fn skill_level_toggle() {
+    assert_eq!(SkillLevel::Simple.is_advanced(), false);
+    assert_eq!(SkillLevel::Detailed.is_advanced(), true);
+}