Sandbox creation dialog wrongly defaults the sandbox name

Tiny thing that's been bugging me: I always have to manually input a Sandbox name when creating a new sandbox, even though there appears to be a name defaulted in the UI

I keep getting annoyed that I have to enter a sandbox name manually. But then I say "well, I only experience this because I'm testing and creating a lot of sandboxes I don't care about. It won't affect users in production."

But that's not quite right! The UI is actually telling me that:

• My Sandbox is the current name
• my-sandbox will be the internal id

And both of these things are lies because My Sandbox is just a placeholder.

The UI is inconsistent with itself. Create is greyed out, but it sort of looks like a name is preset for me.

Suggested fixes here:

• Default the sandbox name to Dev or maybe Staging (to me they mean different things, I don't know what the most useful default is for users_
• And I do mean default, not placeholder
• When the dialog opens, the default value should be focused and ideally the text highlighted - so I can just type.
• When there is no sandbox name entered (which now only happens when the user deletes the text, show a clear error. Ideally this is instead of of the your sandbox will be named... text.

Start at the sandbox creation dialog and inspect how its name field, placeholder, focus behavior, and Create button state are set. Done means the field has a real default value, is focused with its text selected when the dialog opens, and shows a clear error when the value is deleted.