Make appsettings.json optional; support appsettings.local.json override

App no longer crashes on startup when appsettings.json is absent (e.g.
fresh clone). Configuration is loaded from appsettings.json if present,
then overridden by appsettings.local.json if that exists. Both files
are gitignored to keep secrets out of the repo.

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

.gitignore

@@ -7,6 +7,7 @@ obj/
 
 # Config with secrets — never commit
 EbayListingTool/appsettings.json
+EbayListingTool/appsettings.local.json
 
 # Rider / JetBrains
 .idea/