Feature: Add report year customization

[Nik-mmzd]

Allows users to configure the report year in the WebUI or using env var. This will also allow users to run their own Rewind instances earlier than December 31.

[Chaphasilor]

I'll see what I can do. The issue is that the year has to be set when building the website, since there are no env vars in the browser. That would mean re-building the project every time the container starts up (which probably isn't hard, but unconventional).

Maybe a better solution would be to make the year user-selectable somewhere, so that it can be changed dynamically. That might go hand-in-hand with #10 for specifying a target (month, year, multiple years).

[Nik-mmzd]

I think both #12 and #13 can be solved smth like "generate some kind of config json (with URLs, year and so on) on startup and load it from web app"