refactor: remove app::Message type from Graph

I wonder, given the description of canvas::Program,
would the graph be better as a true widget?

I'll have to learn how to actually implement a widget though.

src/app/graph.rs

@@ -2,8 +2,6 @@ use std::collections::VecDeque;
 
 use iced::{Color, Point, Renderer, Theme, mouse, widget::canvas};
 
-use crate::app;
-
 const CYAN: Color = Color::from_rgb8(0, 255, 255);
 
 #[derive(Default)]
@@ -33,7 +31,7 @@ impl Graph {
     }
 }
 
-impl canvas::Program<app::Message, Theme, Renderer> for Graph {
+impl canvas::Program<(), Theme, Renderer> for Graph {
     type State = ();
     fn draw(
         &self,