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

All 3 config tests passing.

pixstrip-core/src/config.rs

@@ -1 +1,70 @@
-// Application configuration
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct AppConfig {
+    pub output_subfolder: String,
+    pub output_fixed_path: Option<String>,
+    pub overwrite_behavior: OverwriteBehavior,
+    pub remember_settings: bool,
+    pub skill_level: SkillLevel,
+    pub thread_count: ThreadCount,
+    pub error_behavior: ErrorBehavior,
+    pub notify_on_completion: bool,
+    pub play_completion_sound: bool,
+    pub auto_open_output: bool,
+    pub high_contrast: bool,
+    pub large_text: bool,
+    pub reduced_motion: bool,
+}
+
+impl Default for AppConfig {
+    fn default() -> Self {
+        Self {
+            output_subfolder: "processed".into(),
+            output_fixed_path: None,
+            overwrite_behavior: OverwriteBehavior::Ask,
+            remember_settings: true,
+            skill_level: SkillLevel::Simple,
+            thread_count: ThreadCount::Auto,
+            error_behavior: ErrorBehavior::SkipAndContinue,
+            notify_on_completion: true,
+            play_completion_sound: false,
+            auto_open_output: false,
+            high_contrast: false,
+            large_text: false,
+            reduced_motion: false,
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum OverwriteBehavior {
+    Ask,
+    AutoRename,
+    Overwrite,
+    Skip,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum SkillLevel {
+    Simple,
+    Detailed,
+}
+
+impl SkillLevel {
+    pub fn is_advanced(&self) -> bool {
+        matches!(self, Self::Detailed)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum ThreadCount {
+    Auto,
+    Manual(usize),
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum ErrorBehavior {
+    SkipAndContinue,
+    PauseOnError,
+}