feat: add high contrast mode for accessibility

- Add HighContrast variant to Theme enum in Rust backend
- Add high-contrast CSS theme with pure black/white for max contrast
- Use bright saturated colors for syntax highlighting
- Add High Contrast button to theme selector with accessibility tooltip
- Follows WCAG guidelines for contrast ratios

Closes #20

src-tauri/src/config.rs

@@ -123,6 +123,8 @@ pub enum Theme {
     #[default]
     Dark,
     Light,
+    #[serde(rename = "high-contrast")]
+    HighContrast,
 }
 
 #[cfg(test)]
@@ -185,8 +187,13 @@ mod tests {
     fn test_theme_serialization() {
         let dark = Theme::Dark;
         let light = Theme::Light;
+        let high_contrast = Theme::HighContrast;
 
         assert_eq!(serde_json::to_string(&dark).unwrap(), "\"dark\"");
         assert_eq!(serde_json::to_string(&light).unwrap(), "\"light\"");
+        assert_eq!(
+            serde_json::to_string(&high_contrast).unwrap(),
+            "\"high-contrast\""
+        );
     }
 }