ychat: fix mobile address-bar clipping and trim online frame

Two refinements to the mobile chat layout in body.chatlayout:

* height: 100dvh (with 100vh kept first as a fallback). 100vh is the
  largest possible viewport, i.e. with the mobile browser's address bar
  hidden; with this layout's overflow:hidden that pushed the bottom row
  (input frame, Send/Select buttons) off-screen whenever the address bar
  was actually showing, which is most of the time. 100dvh tracks the real
  currently-visible viewport instead.

* .online-frame mobile height 90px -> 56px. 90px left a lot of visibly
  empty black space below the room name and a user or two -- most rooms
  don't have enough online users to fill it. 56px fits the room heading
  + 1-2 names; the frame still scrolls internally (iframes do by
  default) if a room gets busier.

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