chore: gitignore viewer build artifacts

Prevent generated .NET bin/obj, dist-browser bundles, node_modules, and .gradle
under viewer/ from being committed (root cause of an accidental bulk-add).

Author: yfedoseev

.gitignore

@@ -455,3 +455,10 @@ r/*.tar.gz
 /elixir/deps/
 /elixir/priv/
 /elixir/mix.lock
+
+# Viewer build artifacts (never commit generated output)
+viewer/dotnet/**/bin/
+viewer/dotnet/**/obj/
+viewer/dist-browser/
+viewer/**/node_modules/
+viewer/**/.gradle/