Settings explorer is incomprehensible, and changing settings doesn't appear to work

### Steps to reproduce

I was excited to discover the settings explorer, but sadly I couldn't make it work.

1. Open dev tools
3. Click "Settings explorer"
4. Search for and open a setting, such as `debug_scroll_panel`
5. Click "Edit values"
6. Observe. Be confused:
7. In "Values at explicit levels", change `"device": null` to `"device": true`,
8. Click "Save setting values"

### Outcome

#### What did you expect?

The setting to have changed

#### What happened instead?

Nothing, afaict.

Furthermore, I really don't understand that UI:
* What is the difference between `room-device` level, and `device` level but "Settable at room"? (I don't really understand the words "Settable at room" anyway, but presumably it means something like "Settable for a specific room"?)
* Why are "default" values shown in the "values at explicit levels" writeables?

### Operating system

_No response_

### Browser information

_No response_

### URL for webapp

_No response_

### Application version

Element version: 1.11.105-dev

### Homeserver

_No response_

### Will you send logs?

No