fix: responsive C4 board on mobile and preserve chat input during morph

Shrink board cells to 36px with tighter gaps on <768px screens so the
board fits on 375px phones. Add DataIgnoreMorph to the C4 chat input
so typing isn't disrupted when new messages arrive.

ui/c4chat.go

@@ -50,7 +50,7 @@ func C4Chat(messages []C4ChatMessage, msgBind, sendClick, sendKeyDown h.H) h.H {
 
 	return h.Div(h.Class("c4-chat"),
 		h.Div(historyAttrs...),
-		h.Div(h.Class("c4-chat-input"),
+		h.Div(h.Class("c4-chat-input"), h.DataIgnoreMorph(),
 			h.Input(
 				h.Type("text"),
 				h.Attr("placeholder", "Chat..."),