allow setting minimum height for graphs

[Roguelazer]

If you make a dashboard with several rows of graphs, it quickly becomes unreadable on smaller screens, since firefly "intelligently" sizes graphs to fit on one screen without scrolling. We should set a minimum height/width for graphs (I would vote for something like 400 pixels) and refuse to scale a graph below that.

Probably worth putting in ui_server.yaml, since it seems like different consumers might have different preferences.

Or we could make it per-client and use that client-side configuration UI that we used to talk about...