docs(node): make quickstart runnable end to end

[JoshKappler]

Type: fix. The Node quickstart cannot be completed as written. A reader who follows it top to bottom hits a hard error on the first code sample and never reaches a running service.

This quickstart was created in the #354 docs refactor merged July 16 and had not been walked end to end yet. The docs bots do mechanical sync and grammar passes (open PR #390 is a grammar-only pass on a sibling quickstart page), so first-run defects like this survive them.

What is broken

Following the page's own steps produces a project whose package.json has "type": "commonjs", which is what npm init -y writes on current npm. The first sample uses ESM import syntax, so node server.js fails immediately:

SyntaxError: Cannot use import statement outside a module

Three more gaps stop the page short of a running service:

• The intro names Express as the target use case and the samples import express, but the install step only installs gt-node and gt.
• No block has an app.listen, and no step runs or checks the server, so even with the imports fixed node server.js exits silently.
• npx gt translate dead-ends: it requires a gt.config.json and credentials that the page never creates, and it points at gt init, which the page never mentions.

What this changes

• Adds a one-line note in step 1 to set "type": "module".
• Adds express to the install commands in all four package managers.
• Adds app.listen to the final server block and a new "Run and verify" step (node server.js plus a curl and the expected response).
• Adds a one-line note at the translate step to run npx gt init first to create gt.config.json and credentials.
• Fixes the code fences that were tagged ts while titled server.js; the code is plain JavaScript.

Verified by scaffolding the page from scratch in a clean directory: npm init -y, set "type": "module", install the dependencies, paste the assembled server code, node server.js, then curl. The server returns HTTP 200 with {"message":"Hello, Alice!"} (the source string, since no credentials are set). The repo CI checks pass locally: unsafe-HTML validation, link validation, and pnpm build:content.

On the obvious counterargument

Node's error message does name the fix for the ESM problem, so an experienced reader could add "type": "module" on their own. That holds for that one line and not for the rest: the missing express install, the missing app.listen, and the gt translate prerequisites give no such hint, and a quickstart should not make the reader debug its first sample. The page still states the ESM line so the happy path runs with no detours.

Coordination with #389

Draft PR #389 is editing this same file's step 2 (credentials and environment variables), blocked on a gt-node release. I kept my scope to page structure and left step 2 untouched. One consequence: the step 2 code block keeps its ts fence, so after this lands the page has one ts-fenced block and two js-fenced blocks. Aligning that last fence belongs with #389's rewrite of that block, so I note it here rather than editing a region #389 is actively changing.

There is also a merge-order interaction. This PR's new "Run and verify" step expects the handler to echo the source string, which is correct on current main where step 2 sets no API key. #389 rewrites step 2 to export a real dev key, and a dev key can trigger on-demand runtime translation instead of a source-string echo, which would make the verify step's expected output wrong. Two clean orders: land #389 first, then a one-line follow-up here adjusting the verify wording; or land this PR first (correct on current main) and make that same one-line adjustment when #389 lands. Either way it is a single follow-up line, not a rebase conflict.

If I have the framing wrong

If the quickstart intentionally assumes the reader already has an Express app and a configured project, I'm happy to rescope to that framing instead, for example dropping the install and run steps and stating the assumption up front.

[JoshKappler]

Folding this into #391 so the quickstart fixes are one review instead of four. The commit is cherry-picked there unchanged and the write-up moved into that PR's body. The #389 coordination note rides along in #391.