style: improve CustomHeader responsiveness for larger screens

- Updated the CustomHeader component to increase the font size on larger screens, enhancing readability and visual hierarchy.

Author: geromegrignon

src/components/ui/CustomHeader.astro

@@ -5,7 +5,7 @@ import ThemeSelect from "@astrojs/starlight/components/ThemeSelect.astro";
 ---
 
 <nav class="flex items-center justify-between">
-<h1 class="font-[Geist] text-base md:text-xl flex items-center">
+<h1 class="font-[Geist] text-base lg:text-xl flex items-center">
      <span>tutorials.</span>
     <span class="text-[#02BCFF]">angular.</span>
     <span>courses</span>