Move low-stock hint to badge, right-align VAT breakdown, gate cart discount field, split billing/shipping delivery method

- Removed the inline "Nur noch wenige verfügbar" text hint from
  AddToCartButton/AddToCartInlineButton (was making card heights vary in
  every grid that renders them — RelatedProducts, ProductSpotlight's CTA
  row) — now only shown via the same image-overlaid pill badge
  Ausverkauft/discount already use (position: absolute, doesn't affect
  layout). Added that badge to RelatedProducts.tsx and todo-cards'
  Pricing.tsx, which didn't have it before.
- RelatedProducts cards now also show "inkl. X% MwSt." (was missing
  entirely)
- VatBreakdown rows are now flex rows with a spacer instead of plain
  text, so every € amount right-aligns to the same edge regardless of
  how many digits the rate itself has (was visibly staggered with mixed
  7%/19% rates)
- Cart's manual discount-code field only renders when Payload actually
  has at least one active code right now (lib/discountServer.ts's new
  hasActiveDiscountCode()) — no point showing an open field that could
  never validate. An already-applied code (e.g. from an older session)
  still always shows its own result row regardless.
- Checkout's "1. Rechnungsadresse" no longer offers a Packstation option
  — a Packstation isn't a valid billing address for an invoice. Only a
  plain street address now; Packstation is only offered on the separate,
  optional "Abweichende Lieferadresse" section, which already had its own
  address/Packstation toggle.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/checkoutDraft.ts

@@ -14,10 +14,10 @@ export type CheckoutDraft = {
   firstName: string;
   lastName: string;
   email: string;
-  deliveryMethod: "address" | "packstation";
+  // Rechnungsadresse is always a plain street address now — no
+  // deliveryMethod/packstationNumber/postNumber here, only on the
+  // shipping* override fields below (see CheckoutContent.tsx).
   street: string;
-  packstationNumber: string;
-  postNumber: string;
   zip: string;
   city: string;
   country: string;

app/lib/discountServer.ts

@@ -44,6 +44,31 @@ async function fetchDiscountCode(code: string): Promise<PayloadDiscountCode | nu
   return data.docs?.[0] ?? null;
 }
 
+// Whether it's worth showing the cart's manual "Rabattcode" input field at
+// all — no point offering an open text field for a shopper to type into
+// when there's nothing in Payload that could ever validate. Existence-only
+// check (active: true), not the fuller validFrom/validUntil/minOrderValue
+// window validateDiscountCode() does for an actual submitted code — this
+// just gates whether the field renders, the real validation still happens
+// at apply time regardless.
+export async function hasActiveDiscountCode(): Promise<boolean> {
+  const params = new URLSearchParams({
+    "where[tenant.slug][equals]": TENANT_SLUG,
+    "where[active][equals]": "true",
+    limit: "1",
+  });
+  const res = await fetch(`${PAYLOAD_URL}/api/discount-codes?${params}`, {
+    headers: { "x-discount-service-secret": SERVICE_SECRET },
+    next: { revalidate: 60 },
+  });
+  if (!res.ok) {
+    console.error(`hasActiveDiscountCode: Payload returned ${res.status} ${res.statusText}`);
+    return false;
+  }
+  const data: { docs?: unknown[] } = await res.json();
+  return (data.docs?.length ?? 0) > 0;
+}
+
 export type DiscountValidation =
   | { valid: true; doc: PayloadDiscountCode }
   | { valid: false; reason: string };