Concatenate strings in jq

My input looks like:

{"font1": {"charInfo": [            {"char": "a","other_stuff": 12            },            {"char": "b","other_stuff": 63            }        ]    },"font2": {"charInfo": [            {"char": "s","other_stuff": 1            },            {"char": "x","other_stuff": 4            }        ]    }}

and I'd like to aggregate (ideally concatenate) the "char"-s per font:

{"font1": "ab","font2": "sx"}

How can achieve this using jq?

This is what I have so far:

jq '. | keys[] as $font | {font: $font} + .[$font] | {font:.font, chars:[.charInfo[].char]|join("")}'enter code here

Which brings it to:

{"font": "font1","chars": "ab"}{"font": "font2","chars": "sx"}

but I can't find how I turn the "font" to a key of a map