refactor(model): rename Yavsc.Blogspot.BlogPost to BlogPostDto

When commit 0e95e283 moved BlogPost from PostIt.Models to
Yavsc.Blogspot, it created an unfortunate collision with the
server-side EF entity Yavsc.Models.Blog.BlogPost. The two
classes have nothing in common beyond the name; the DTO is
the wire shape PostIt exchanges with the Blogs API, the EF
entity is the persistence model. Server code that imports both
namespaces (BlogSpotService.cs, etc.) ended up with 'BlogPost
is an ambiguous reference between X and Y' errors.

Renaming the client DTO to BlogPostDto (matching the
naming convention of the other DTOs in Yavsc.Api.Client.Dtos
— CircleDto, CircleAuthorizationDto, UserSearchResultDto)
disambiguates without renaming the EF entity on the server.

The namespace stays Yavsc.Blogspot; only the class name
changes. All call sites (client code, tests, XAML DataTemplates,
XML doc comments) are updated mechanically.

src/PostIt.Tests/BearerScopeTests.cs

@@ -97,7 +97,7 @@ public class BearerScopeTests
             // CapturingHttpHandler is the assertion point. It
             // records the first request's Authorization header and
             // returns 200 with an empty array (BlogApiClient
-            // deserialises to List<BlogPost>).
+            // deserialises to List<BlogPostDto>).
             var captured = new CapturingHttpHandler();
             var client = new YavscApiClient(
                 settings,