Use the correct Antananarivo aerial image, lift plane to upper half

Background was inadvertently the wrong Gemini export (an unrelated
airliner-over-mountains photo) — sorted out and replaced with the
intended aerial illustration of Antananarivo (Lake Anosy + Rova
hill + city, 487 KB, 1920px wide).

Also lifted the plane's trajectory: y goes from +7 (offscreen
upper-left) to +2 (still upper half, exiting right) instead of
+5..-2. The plane now stays clearly in the upper third of the
viewport, leaving room for the centered CTA button and the city
detail at the bottom.

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

js/intro-scene.js

@@ -107,7 +107,7 @@ function tick() {
      - mouse.x = 1 → sortie en bas-droite (hors champ)
   */
   const px = -16 + mouse.x * 32;          // -16 à +16
-  const py = 5 - mouse.x * 7;             // descend de +5 à -2 selon X
+  const py = 7 - mouse.x * 5;             // descend de +7 à +2 (haut de l'écran)
   /* Léger bobbing autonome pour qu'il reste vivant même sans bouger la souris */
   const bob = Math.sin(t * 0.9) * 0.12;