Don't leak .env file into frontend bundle

[naftis]

1. Import all the env's frontend needs in vite.config.ts define.
2. Don't import them from shared, but directly refer to them.
3. Disallow importing @shared/config, and add a ESLint rule to tell the user why not.

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..or..

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1. Start using VITE_ -prefixed env variables in shared/config.ts
2. Import them directly in the frontend

note, both solutions need to have dotenv() configured in vite.config.ts, to load .env file on build.

[eioo]

3rd option, same as 2nd one but change the VITE_ prefix to something more fitting: https://vitejs.dev/config/#envprefix