feat(tui): add manual Y-axis scaling and improve graph axis logic

Introduce a toggleable graph scale (Auto, 100ms–2s) via the 's' key to provide better control over visualization ranges and implement smarter Y-axis bounds that handle signed versus non-negative metrics.

crates/irtt-cli/src/cmd/tui/run.rs

@@ -322,6 +322,10 @@ fn handle_input(state: &mut TuiState, shutdown_requested: &AtomicBool) -> io::Re
                 state.cycle_graph_mode();
                 force_render = true;
             }
+            KeyCode::Char('s') => {
+                state.cycle_graph_scale();
+                force_render = true;
+            }
             KeyCode::Char('f') => {
                 state.toggle_full_graph();
                 force_render = true;