Document GraphQL subscriptions with Apollo (Pro-first)

[irinanazarova]

Adds a client-facing GraphQL page to the JavaScript sidebar group: Apollo GraphQLWsLink setup against the Pro GraphQL endpoint, connection-params and JWT auth, server requirements, and a one-line pointer to the OSS ActionCableLink route; cross-linked with the server Apollo page.

[irinanazarova]

Self-review on 4250c6f; one fix applied in 4eb7ffe.

Accuracy

🔴 Broken anchor to the Action Cable migration section (fixed)

The "Without Pro" sentence linked client-side.md#migrate-from-railsactioncable, but the built HTML slug is migrate-from-rails-actioncable (VitePress hyphenates at the / in "@rails/actioncable"). Lychee passed because it does not validate fragments, so CI green masked it. Verified against the built HTML after the fix.

Spot-checks that passed

• 🟢 Apollo snippet is current: the live Apollo docs still recommend GraphQLWsLink from @apollo/client/link/subscriptions with graphql-ws; the pattern has not changed.
• 🟢 ActionCableLink({ cable }) accepting a consumer is documented on graphql-ruby's Apollo subscriptions page, so the "Without Pro" pointer is accurate.
• 🟢 Server facts match the options reference: --graphql_path (enables the proxy), --graphql_channel default GraphqlChannel, --graphql_action default execute.
• 🟢 Auth claims (x-apollo-connection JSON header, jid connection param or query param for JWT) match the existing anycable-go/apollo.md, which remains the canonical server page and now cross-links here.
• 🟢 Rendered the built page in a browser: layout, code blocks, and sidebar entry all correct; the page lands in llms.txt.

Site-wide observation (out of scope here)

<p class="pro-badge-header"></p> is dormant on every Pro page: the VitePress theme defines only .pro-badge (an unused image style) and nothing targets pro-badge-header, so no Pro badge renders anywhere on the site. This page follows the existing convention and carries the Pro framing in its first sentence, but the badge deserves a separate theme fix.

Recommendation

Ship it. The one broken anchor is fixed and verified against the built HTML.