Add unit tests for utils/og.ts

[Newtdev]

`packages/referral-web/src/utils/og.ts` has no test coverage. It's small (35 lines) and generates Open Graph meta tags for the landing page.

Part of closing `referral-web`'s test coverage gap — see the sibling issues for `platformDetect.ts`, `storeUrls.ts`, and `clipboardHandoff.ts`.