[Feature] Configure multiple ArchiveBox instances

## Use case

I want to manage 2 differents ArchiveBox instances:
- A public one
- A private one

And of course add links to them via the browser extension.

But we can't install multiple instances of an extension on a browser, so I can configure only one instance at a time to add URL to archive.

## Suggestion

Permit to register multiple `Base URL` in the extension configuration, associated with a `name`.

For example:
- `name`: `Instance 1`
- `Base URL`: `https://instance1.example.org`
- `name`: `Instance 2`
- `Base URL`: `https://instance2.example.org`

Then, when we right-clic on a webpage, on hover of the `ArchiveBox Exporter` menu, we can select options like:
- `[Instance 1] Archive Current Page`
- `[Instance 1] Add Current Domain to List`
- `[Instance 2] Archive Current Page`
- `[Instance 2] Add Current Domain to List`

Of course if only one instance is configured, we can keep showing `Archive Current Page` and `Add Current Domain to List` without mentioning instance name to don't break user's mental pattern.

Start by locating the extension configuration and the ArchiveBox Exporter context-menu entry points. Trace how the current Base URL is stored and how the two existing actions are registered. Done means multiple named instances can be configured, each action targets the selected instance, and single-instance labels remain unchanged.

Written by the indexing model from the issue text.