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/ui/theme-toggle.tsx

@@ -0,0 +1,71 @@
+"use client";
+
+import * as React from "react";
+import { Moon, Sun } from "lucide-react";
+import { useTheme } from "next-themes";
+
+import { Button } from "@/components/ui/button";
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuTrigger,
+} from "@/components/ui/dropdown-menu";
+
+export function ThemeToggle() {
+  const { setTheme } = useTheme();
+
+  return (
+    <DropdownMenu>
+      <DropdownMenuTrigger asChild>
+        <Button variant="ghost" size="sm" className="h-8 w-8 px-0">
+          <Sun className="rotate-0 scale-100 transition-all dark:-rotate-90 dark:scale-0" />
+          <Moon className="absolute rotate-90 scale-0 transition-all dark:rotate-0 dark:scale-100" />
+          <span className="sr-only">Toggle theme</span>
+        </Button>
+      </DropdownMenuTrigger>
+      <DropdownMenuContent align="end">
+        <DropdownMenuItem onClick={() => setTheme("light")}>
+          Light
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme("dark")}>
+          Dark
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme("system")}>
+          System
+        </DropdownMenuItem>
+      </DropdownMenuContent>
+    </DropdownMenu>
+  );
+}
+
+export function SimpleThemeToggle() {
+  const { theme, setTheme } = useTheme();
+  const [mounted, setMounted] = React.useState(false);
+
+  React.useEffect(() => {
+    setMounted(true);
+  }, []);
+
+  if (!mounted) {
+    return (
+      <Button variant="ghost" size="sm" className="h-8 w-8 px-0">
+        <Sun className="h-4 w-4" />
+        <span className="sr-only">Toggle theme</span>
+      </Button>
+    );
+  }
+
+  return (
+    <Button
+      variant="ghost"
+      size="sm"
+      className="h-8 w-8 px-0"
+      onClick={() => setTheme(theme === "dark" ? "light" : "dark")}
+    >
+      <Sun className="h-4 w-4 rotate-0 scale-100 transition-all dark:-rotate-90 dark:scale-0" />
+      <Moon className="absolute h-4 w-4 rotate-90 scale-0 transition-all dark:rotate-0 dark:scale-100" />
+      <span className="sr-only">Toggle theme</span>
+    </Button>
+  );
+}