style: improve CSS organization and add state components

CSS improvements:
- Remove duplicate CSS reset from styles-modern.css
- Add focus-visible styles for sidebar icons
- Add error/loading state components (skeleton, spinner, messages)
- Add success, warning, info message components
- Add dark theme support for new components

Code quality:
- Replace inline error style with CSS class in renderer.js

Amit Haridas

src/styles-modern.css

@@ -39,13 +39,7 @@
   --transition-ease: cubic-bezier(0.4, 0, 0.2, 1);
 }
 
-/* Reset & Base */
-* {
-  margin: 0;
-  padding: 0;
-  box-sizing: border-box;
-}
-
+/* Base styles - Reset is in styles.css */
 body {
   font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
   overflow: hidden;