feat: implement server-side pricing calculation and add site settings management

Co-authored-by: Copilot <copilot@github.com>

frontend/app.js

@@ -522,3 +522,11 @@ langToggle.textContent = getLang() === "de" ? "EN" : "DE";
 applyI18n();
 renderReviews();
 loadVehicles();
+
+// Load hero image from site_settings
+(async () => {
+    const { data } = await supabase.from("site_settings").select("value").eq("key", "hero_image_url").single();
+    if (data && data.value) {
+        document.querySelector(".hero").style.setProperty("--hero-bg", `url('${data.value}')`);
+    }
+})();