PostIt: fix blog API double-prefix; drop redundant New

BlogApiClient's "api/blog" path combined with the BaseAddress's
"api/v1/" prefix to produce 404s on every call. Drop the redundant
"api/" segment, let Save handle the create case (no selection) and
remove the now-redundant New button + command.

src/PostIt/PostIt/Services/BlogApiClient.cs

@@ -22,7 +22,7 @@ namespace PostIt.Services;
 /// </summary>
 public sealed class BlogApiClient
 {
-    private const string DefaultPathPrefix = "api/blog";
+    private const string DefaultPathPrefix = "blog";
 
     private readonly YavscApiClient _api;
     private readonly string _pathPrefix;