API/Wallet version mismatch detection

[labs-dlugo]

• Use package.json's major version digit. We are not using that digit for anything else since it's not a library.
• Obviously respect Semantic Versioning from now on.
• If API's major version digit does not match Wallet's, reject the connection.

I did explore some Wallet/API version matching through api-schema.ts in shock-common. Allows for more granular control but would require better GIT discipline (so shock-common v2 can get a non-breaking fix that also shock-common v3 gets).

Still not sure which path is better but API versions are a thing since always.

Lots of authorities do recommend API versioning through a header and not through URLs (e.g. foo.com/v1/posts) and I think it's a good fit (and easier to implement) for our use case.