One Click to join all subspaces

### Your use case

#### What would you like to do?
When I am in a Big Space like https://matrix.to/#/#fosdem2024:fosdem.org , i Would be able to let element join all subspaces.

#### Why would you like to do it?
Because rooms you joined which are in subspaces aren't sorted in the main space and it's really hard joining all the subspaces manual

#### How would you like to achieve it?
A Join Button by Rooms and Spaces
When the Button is clicked it asked if you want to join subspaces or subspaces and rooms.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

No files, tests, or code entry points are mentioned. Start by reviewing Element Web's existing space and room join flows alongside the linked screenshot; define the confirmation behavior for joining subspaces alone versus subspaces and rooms, and verify the resulting navigation and membership behavior.

Written by the indexing model from the issue text.