Custom `Home` space

### Your use case

#### What would you like to do?

I want to set a space as the "home":

- At startup, the app auto-opens the space.
- If I am in another space and press the Back button, the app returns to my "home" space.

#### Why would you like to do it?
Element wants me to organize the rooms I use the most in spaces. What Element currently calls "home" is actually the list of rooms that are not in a space, so naturally they are not my mostly used rooms (on the contrary).

(edit) This is also inconvenient. I am used to quitting apps with the Back button, but if I do this in Element, I will have to select my favorite space again the next time I open the app.

#### How would you like to achieve it?
Selecting the space in the app settings, or a checkbox `Use as home` in the space settings.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

### Are you willing to provide a PR?

No