fix: Minor UI improvements to layout and LengthInput

Removes unnecessary top row link and adds default placeholder to
LengthInput component.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CutList.Web/Components/Layout/MainLayout.razor

@@ -6,10 +6,6 @@
     </div>
 
     <main>
-        <div class="top-row px-4">
-            <a href="https://github.com/yourusername/cutlist" target="_blank">CutList Web</a>
-        </div>
-
         <article class="content px-4">
             @Body
         </article>