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

components/ResponseTimeDistribution.tsx

@@ -114,7 +114,12 @@ export default function ResponseTimeDistribution({
           />
           <Tooltip content={<CustomTooltip />} />
 
-          <Bar dataKey="value" radius={[4, 4, 0, 0]} fill="hsl(var(--chart-1))">
+          <Bar 
+            dataKey="value" 
+            radius={[4, 4, 0, 0]} 
+            fill="hsl(var(--chart-1))"
+            maxBarSize={60}
+          >
             {chartData.map((entry, index) => (
               <Bar key={`cell-${index}`} fill={entry.color} />
             ))}