fix(storybook): silence NODE_ENV DefinePlugin warning; revert CI memory cap to 4g

.storybook/main.ts: webpack core's auto-DefinePlugin (from optimization.nodeEnv)
and Storybook's own preset both define process.env.NODE_ENV on the same
compile, triggering a "Conflicting values" warning even though both resolve
to "development" locally. Disable the redundant one via webpackFinal.

ci.yml: revert the storybook-a11y container's memory cap 6g -> 4g. The 6g
bump was based on an unconstrained local RSS measurement (~5.8GB) that
doesn't reflect real behavior under a cgroup cap. Verified directly: running
this job's exact steps (npm ci, playwright install, build-storybook,
test-storybook:ci) in `docker run --cpus=2 --memory=4g --memory-swap=4g
node:24-bookworm` completes clean, no OOM, 62/62 suites passing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.storybook/main.ts

@@ -17,5 +17,12 @@ const config: StorybookConfig = {
   // Serve the vendored CIBG package so preview-head.html can <link> its CSS (and its
   // relative font/icon/image url()s resolve) — mirrors index.html for the real app.
   staticDirs: ['../public'],
+  // Webpack's own auto-DefinePlugin (from `optimization.nodeEnv`) and Storybook's preset both
+  // define `process.env.NODE_ENV` on the same compile, which webpack warns about as
+  // "Conflicting values" even though they resolve to the same string. Disable the redundant one.
+  webpackFinal: async (config) => {
+    config.optimization = { ...config.optimization, nodeEnv: false };
+    return config;
+  },
 };
 export default config;