fix: improve dark mode compatibility and chart visibility

- Fix TopQuestionsChart with proper dark mode colors using CSS variables and shadcn/ui components
- Enhance ResponseTimeDistribution with thicker bars (maxBarSize: 60)
- Replace GeographicMap with dark/light mode compatible CartoDB tiles
- Add custom text selection background color with primary theme color
- Update all loading states to use proper CSS variables instead of hardcoded colors
- Fix dashboard layout background to use bg-background instead of bg-gray-100

app/dashboard/layout.tsx

@@ -57,7 +57,7 @@ export default function DashboardLayout({ children }: { children: ReactNode }) {
   }
 
   return (
-    <div className="flex h-screen bg-gray-100">
+    <div className="flex h-screen bg-background">
       <Sidebar
         isExpanded={isSidebarExpanded}
         isMobile={isMobile}