After saving, show "Saved! Add another from this activity?" with two
buttons: "Add another" (resets name/handles, keeps map loaded) and
"Done" (navigates to /segments/).
Shows a dim area for the visible range around the selection (4% padding)
and a blue overlay for the selected segment, with a light stroke on the
top edge. Both the x-domain and y-domain track the selection, so the
chart zooms in as the handles narrow. Elevation min/max labels overlaid
at top-left and bottom-left.