Untrack client/node_modules and gitignore it

node_modules was vendored into the repo, which caused platform-specific
rollup/esbuild binary churn on every local install. Dependencies are
installed fresh during the Docker build, so there's no reason to track them.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -1 +1,4 @@
 .claude/
+
+# Dependencies are installed fresh in the Docker build; never vendor them in git.
+node_modules/