fix(docker): serve .mjs modules as javascript

[Ryback2501]

Summary

• Fix the SPA being completely non-functional when served from the Docker image: nginx served .mjs ES modules as application/octet-stream, so browsers refused to execute them, the vendor crypto imports failed, and app.js never initialized (every button, including "Generate new key pair", was dead).

Changes

Container

• Dockerfile — register .mjs as application/javascript in nginx's mime.types (it has no .mjs entry by default). All vendor/*.mjs files now serve with a JavaScript content type.

CI

• .github/workflows/ci-jobs.yml — add a docker-smoke step that fetches vendor/secp256k1.mjs and asserts its Content-Type is JavaScript, so this regression can't recur.

Test plan

• Build the image and confirm curl -I .../vendor/secp256k1.mjs returns Content-Type: application/javascript.
• Load the app from the container in a browser, click "Generate new key pair", and confirm a key pair is created (no module errors in the console).