[FEAT] Make app title configurable (white-labeling)

[FunctionDJ]

Regardless of how the technical (#57) and legal aspects of publicly hosting BetterBahn instances pan out, there's a clear desire of users to do so.

In order to help with avoiding confusiong between third-party ("unofficial") instances of BetterBahn with services run by the BetterBahn project, I suggest we add "white-labeling" capabilities to the app.

In practise that means making the following things configurable:

• the <title> element (handled by Next.js metadata export)
• the heading of the app
• any other text instances of "BetterBahn" that, by context, refer to the instance instead of the software project

All of these can probably share the same parameter, which could be made configurable through:

• the .env.production file for deployments without Docker
• env parameters for docker run or docker compose

I strongly suggest to make this parameter mandatory to configure by making the app fail to launch with the parameter missing or empty. The templates for these files should include a notice that public instances should not use "BetterBahn" alone as their name in accordance to information on relevant GitHub repository documents or https://betterbahn.de as soon as that's present.