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-sidebar.css

@@ -57,6 +57,11 @@
     box-shadow: inset 3px 0 0 var(--primary-dark, #5661b3);
 }
 
+.sidebar-icon:focus-visible {
+    outline: 2px solid var(--primary-dark, #5661b3);
+    outline-offset: 2px;
+}
+
 .sidebar-panel {
     width: 280px;
     background: var(--gray-50, #f9fafb);