Improve usability of personal-spaces-as-room-organizers — "Add to space" context menu action

### Your use case

#### What would you like to do?

When creating a space in Element, the second level "wizard" question (Who are you working with?) offers two suggestions: "Me and my teammates" and "Just me" (with the subheading of "A private space to organize your rooms"). As such, I understand that personal-spaces-as-room-organizers is a supported concept.

Within this concept, I wish that there were simpler, more streamlined ways of adding a room to such a "personal space". Currently, adding a room to a space consists of these steps:

- go to the target space
- click "add"
- click "existing room"
- type the room name from memory(!)
- click "add"
- return back to the room

#### Why would you like to do it?

Usability of personal spaces as a room organization tool would be significantly improved by adding a more streamlined way to add rooms to spaces.

#### How would you like to achieve it?

I'd prefer if there was an "Add to space" submenu in the drop-down context menu of a room, akin to Telegram's "Add to folder" submenu.

Eligible spaces could be limited to invite-only spaces that have this account as the only member (with the menu either disappearing if there were no such spaces, or offering to create a new one, depending on how discoverable we want to make such a feature). In future, spaces could grow an explicit flag that marks or unmarks them as "personal organizers", not unlike the DM flag on regular rooms.

Additionally, support for drag-and-drop between a room and a space could be useful (speaking for myself, I was expecting it to be there).

### Have you considered any alternatives?

I do not see any viable alternatives.

### Additional context

_No response_

No files, tests, or client entry points are named. Start by locating the room context-menu implementation and the logic for identifying eligible personal spaces. Done should include a usable "Add to space" action for the defined spaces, with behavior specified for accounts that have none.

Written by the indexing model from the issue text.