Image manager: cleaned up style changes, dark mode support

- Updated tab handling to be smarter on initial tab selection, to first
  target non-hidden tab panels where they may be handled server-side.
- Extracted contained search box handling styles to _forms.scss, after
  merging with image-manager-specific styles since this is only usage of
  contained variant.
- Aligned focus handling on image manager UI elements.

resources/js/components/tabs.js

@@ -70,7 +70,8 @@ export class Tabs extends Component {
     }
 
     activate() {
-        this.show(this.panels[0].id);
+        const panelToShow = this.panels.find(p => !p.hasAttribute('hidden')) || this.panels[0];
+        this.show(panelToShow.id);
         this.tabList.toggleAttribute('hidden', false);
     }