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>

CutList.Web/Components/Shared/LengthInput.razor

@@ -5,7 +5,7 @@
            class="form-control @(HasError ? "is-invalid" : "")"
            value="@DisplayValue"
            @onchange="OnInputChange"
-           placeholder="e.g., 12' 6" or 144" />
+           placeholder="@Placeholder" />
     @if (HasError)
     {
         <div class="invalid-feedback">@ErrorMessage</div>
@@ -20,7 +20,7 @@
     public EventCallback<decimal> ValueChanged { get; set; }
 
     [Parameter]
-    public string? Placeholder { get; set; }
+    public string Placeholder { get; set; } = "e.g., 12' 6\" or 144";
 
     private string DisplayValue { get; set; } = string.Empty;
     private bool HasError { get; set; }