fix(terrain): align physics body with drawn terrain line

Correct the rectangle center offset calculation in Terrain to account
for both X and Y components perpendicular to each segment. Previously,
only the Y offset was applied, causing physics bodies to shift sideways
on sloped terrain. This fix resolves the bus visibly riding above or
through the ground on non-flat segments.

Also adds background and logo assets.

src/entities/Terrain.js

@@ -66,8 +66,19 @@ export default class Terrain {
       const length = Math.hypot(dx, dy);
       if (length < 1) continue;
 
+      // The rectangle's top edge needs to sit exactly on the a-b line, which
+      // means offsetting its center from the segment's own midpoint by
+      // TERRAIN_DEPTH/2 in the direction PERPENDICULAR to the segment - both
+      // an X and a Y component (-sin/cos of the segment's angle), not just
+      // straight down in world Y. Omitting the X term (as a prior version of
+      // this did) left every non-flat segment's physics body shifted
+      // sideways from its drawn line by TERRAIN_DEPTH/2 * sin(angle) - with
+      // TERRAIN_DEPTH deliberately huge (deep enough nothing ever tunnels
+      // through the bottom), that shift is large even at modest slopes,
+      // which is exactly what made the bus visibly ride above/through the
+      // drawn ground on anything but flat terrain.
       const angle = Math.atan2(dy, dx);
-      const midX = (a.x + b.x) / 2;
+      const midX = (a.x + b.x) / 2 - (TERRAIN_DEPTH / 2) * Math.sin(angle);
       const midY = (a.y + b.y) / 2 + (TERRAIN_DEPTH / 2) * Math.cos(angle);
 
       const body = this.scene.matter.add.rectangle(midX, midY, length, TERRAIN_DEPTH, {