style: adjust visibility of elements in CustomHeader for improved responsiveness

- Updated the CustomHeader component to hide the "angular." text on smaller screens, enhancing the layout for mobile users.

Author: geromegrignon

src/components/ui/CustomHeader.astro

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