Function paint_loading_bar 

pub fn paint_loading_bar(
    buf: &mut [u8],
    screen_w: usize,
    screen_h: usize,
    frac: f32,
)

Draw a horizontal progress bar at the bottom of the screen.

frac ranges from 0.0 (empty) to 1.0 (full). The filled portion uses [BAR_FILL_COLOR]; the remaining track uses [BAR_TRACK_COLOR].