fix(developers): distinguish API errors from taken usernames in availability check

CheckUsernameAsync returned false (taken) on any API failure, making every
username appear taken when SilverDESK was unreachable. Now returns nullable
bool so errors show a warning instead of blocking submission.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

BlazorApp/Endpoints/DeveloperEndpoints.cs

@@ -12,7 +12,9 @@ public static class DeveloperEndpoints
         group.MapGet("/check-username/{username}", async (string username, DeveloperApplicationService service) =>
         {
             var available = await service.CheckUsernameAsync(username);
-            return Results.Ok(new { available });
+            if (available is null)
+                return Results.Problem("Unable to verify username availability", statusCode: 503);
+            return Results.Ok(new { available = available.Value });
         });
 
         group.MapPost("/apply", async (DeveloperApplication application, DeveloperApplicationService service) =>