Fix green bar at bottom of map caused by off-by-one in map height

map_h was subtracting MARGIN twice but the map_area only has a single
bottom margin, leaving the last 8 rows of the land background unrendered.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/render.rs

@@ -167,7 +167,7 @@ pub fn render_frame(
     let map_area = root.margin(TITLE_H, MARGIN, MARGIN, LEGEND_W + MARGIN);
 
     let map_w = IMG_WIDTH - LEGEND_W - MARGIN * 2;
-    let map_h = IMG_HEIGHT - TITLE_H - MARGIN * 2;
+    let map_h = IMG_HEIGHT - TITLE_H - MARGIN;
 
     map_area.fill(&LAND_BG).context("fill map area")?;