System modal dialog fixes
We broke app-auth dialogs (e.g. portal's permission prompt with 5f7d38bd but only since app-auth was just lucky to get it right before. Fix this, a leak in app-auth-prompt and clean up a bit around the edges.
I've moved the system-modal dialog related style changes here too:
- always add a top and bottom margin so the top bar / home bar isn't occluded by dialog content
- fix padding around title
/cc @snwh 2. is the bit from system-modal related bit from !1245 (merged) and and 1. can be seen in the CI screenshots if you compare to 0.27.0.