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

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

frontend/admin.js

@@ -177,6 +177,7 @@ const tabPanels   = {
     customers: document.querySelector("#tab-customers"),
     orders:    document.querySelector("#tab-orders"),
     vehicles:  document.querySelector("#tab-vehicles"),
+    settings:  document.querySelector("#tab-settings"),
 };
 let activeTab = "leads";
 
@@ -206,6 +207,7 @@ function renderActiveTab() {
     if (activeTab === "customers") renderCustomers();
     if (activeTab === "orders")    renderOrders();
     if (activeTab === "vehicles")  renderVehicles();
+    if (activeTab === "settings")  renderSettings();
 }
 
 // Sub-tabs (active/inactive leads)
@@ -1100,4 +1102,47 @@ leadDialog.addEventListener("close", onDialogClose);
 customerDialog.addEventListener("close", onDialogClose);
 orderDialog.addEventListener("close", onDialogClose);
 
+// =========================================================================
+// SETTINGS
+// =========================================================================
+const heroPreview    = document.querySelector("#heroPreview");
+const heroImageInput = document.querySelector("#heroImageInput");
+const heroFeedback   = document.querySelector("#heroFeedback");
+
+async function renderSettings() {
+    const { data } = await supabase.from("site_settings").select("value").eq("key", "hero_image_url").single();
+    const url = data?.value || "/images/ferrari-main-car.png";
+    heroPreview.style.backgroundImage = `url('${url}')`;
+}
+
+heroImageInput.addEventListener("change", async () => {
+    const file = heroImageInput.files?.[0];
+    if (!file) return;
+    heroFeedback.className = "form-feedback";
+    heroFeedback.textContent = "Uploading...";
+    try {
+        const ext = (file.name.split(".").pop() || "jpg").toLowerCase();
+        const path = `site/hero.${ext}`;
+        const { error: upErr } = await supabase.storage
+            .from("vehicle-photos")
+            .upload(path, file, { contentType: file.type, upsert: true });
+        if (upErr) throw upErr;
+        const { data: pub } = supabase.storage.from("vehicle-photos").getPublicUrl(path);
+        const publicUrl = pub.publicUrl;
+
+        // Save to site_settings
+        const { error: dbErr } = await supabase
+            .from("site_settings")
+            .upsert({ key: "hero_image_url", value: publicUrl, updated_at: new Date().toISOString() }, { onConflict: "key" });
+        if (dbErr) throw dbErr;
+
+        heroPreview.style.backgroundImage = `url('${publicUrl}')`;
+        heroFeedback.className = "form-feedback";
+        heroFeedback.textContent = "Gespeichert.";
+    } catch (err) {
+        heroFeedback.className = "form-feedback error";
+        heroFeedback.textContent = err.message || String(err);
+    }
+});
+
 bootstrap();