Amplify: install devDependencies during build (Tailwind/PostCSS need them)

@tailwindcss/postcss lives in devDependencies (along with the rest of
the PostCSS toolchain). When NODE_ENV=production is set in the Amplify
build environment, npm ci skips devDependencies — and next build then
fails resolving @tailwindcss/postcss while compiling globals.css.

amplify.yml now passes --include=dev to npm ci so the build always
installs everything regardless of NODE_ENV. AMPLIFY_DEPLOY.md updated
to warn against setting NODE_ENV=production in the Amplify env vars
panel — it's redundant (Next sets it correctly) and an easy footgun.

amplify.yml

@@ -5,7 +5,10 @@ applications:
         preBuild:
           commands:
             - nvm use $(cat .nvmrc) || nvm install $(cat .nvmrc)
-            - npm ci --cache .npm --prefer-offline
+            # --include=dev is required because the build needs PostCSS /
+            # Tailwind plugins which live in devDependencies; without it,
+            # NODE_ENV=production in the Amplify env causes npm to skip them.
+            - npm ci --include=dev --cache .npm --prefer-offline
         build:
           commands:
             - npm run build