Fix out-of-stock CTA: replace button instead of stacking, collapse notify form

Two problems in the previous back-in-stock commit: the disabled
"Ausverkauft" button and NotifyMeForm stacked, making out-of-stock
cards visibly taller than in-stock siblings — and since ProductGrid's
cards rely on plain CSS Grid row-stretch for equal card height, that
extra height stretched sibling cards and pushed their own buttons
down (screenshot: "In den Warenkorb" CTAs misaligned across a row).

NotifyMeForm now replaces the button slot entirely when out of stock
(matching the height of a normal button when collapsed), and only
expands to the email input after a click.

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

app/components/AddToCartButton.tsx

@@ -88,7 +88,9 @@ export function AddToCartButton({
     : added
       ? "border border-success! bg-success-subtle! hover:bg-success-subtle! text-success!"
       : "";
-  const displayLabel = currentlyOutOfStock ? "Ausverkauft" : limitReached ? "Maximale Menge im Warenkorb" : label;
+  // currentlyOutOfStock has no branch here — that state renders
+  // NotifyMeForm instead of this button entirely (see below).
+  const displayLabel = limitReached ? "Maximale Menge im Warenkorb" : label;
 
   return (
     // Low stock is deliberately NOT surfaced here as its own text line
@@ -117,47 +119,48 @@ export function AddToCartButton({
           ))}
         </select>
       )}
-      <button
-        ref={buttonRef}
-        type="button"
-        onClick={handleClick}
-        disabled={disabled}
-        className={`${base} ${stateClasses}`}
-      >
-        {/* CSS-grid text-stack, not just swapping the button's text node
-            directly — this button is inline-flex/content-sized (no w-full),
-            so "Hinzugefügt ✓" being shorter than most labels made the whole
-            button visibly shrink while showing the success state. Stacking
-            both possible texts in the same grid cell (both invisible ones
-            still contribute to sizing) reserves width for whichever is
-            wider, so the button's box never changes size either way. Now
-            also reserves space for "Ausverkauft"/"Maximale Menge im
-            Warenkorb" — the widest of the four wins regardless of which is
-            showing. */}
-        {/* whitespace-nowrap — inherited by every stacked span below. On a
-            w-full button (e.g. this page's mobile layout), "Maximale Menge
-            im Warenkorb" is long enough to wrap to two lines without this,
-            and since every stacked span shares the same grid cell, that
-            inflated the row height for whichever text is actually showing
-            too — "Ausverkauft" rendered with a tall empty gap underneath it
-            (fixed 2026-07-24). */}
-        <span className="relative grid whitespace-nowrap">
-          <span className="invisible [grid-area:1/1]" aria-hidden="true">
-            {label}
+      {currentlyOutOfStock ? (
+        // Replaces the button slot entirely rather than stacking below a
+        // disabled "Ausverkauft" button — same reasoning as
+        // AddToCartInlineButton's identical swap.
+        <NotifyMeForm productId={numericId} variantName={variants.length > 0 ? (selectedVariant ?? "") : ""} />
+      ) : (
+        <button
+          ref={buttonRef}
+          type="button"
+          onClick={handleClick}
+          disabled={disabled}
+          className={`${base} ${stateClasses}`}
+        >
+          {/* CSS-grid text-stack, not just swapping the button's text node
+              directly — this button is inline-flex/content-sized (no w-full),
+              so "Hinzugefügt ✓" being shorter than most labels made the whole
+              button visibly shrink while showing the success state. Stacking
+              both possible texts in the same grid cell (both invisible ones
+              still contribute to sizing) reserves width for whichever is
+              wider, so the button's box never changes size either way. Now
+              also reserves space for "Maximale Menge im Warenkorb" — the
+              widest of the three wins regardless of which is showing. */}
+          {/* whitespace-nowrap — inherited by every stacked span below. On a
+              w-full button (e.g. this page's mobile layout), "Maximale Menge
+              im Warenkorb" is long enough to wrap to two lines without this,
+              and since every stacked span shares the same grid cell, that
+              inflated the row height for whichever text is actually showing
+              too (fixed 2026-07-24). */}
+          <span className="relative grid whitespace-nowrap">
+            <span className="invisible [grid-area:1/1]" aria-hidden="true">
+              {label}
+            </span>
+            <span className="invisible [grid-area:1/1]" aria-hidden="true">
+              Hinzugefügt ✓
+            </span>
+            <span className="invisible [grid-area:1/1]" aria-hidden="true">
+              Maximale Menge im Warenkorb
+            </span>
+            <span className="[grid-area:1/1]">{added ? "Hinzugefügt ✓" : displayLabel}</span>
           </span>
-          <span className="invisible [grid-area:1/1]" aria-hidden="true">
-            Hinzugefügt ✓
-          </span>
-          <span className="invisible [grid-area:1/1]" aria-hidden="true">
-            Ausverkauft
-          </span>
-          <span className="invisible [grid-area:1/1]" aria-hidden="true">
-            Maximale Menge im Warenkorb
-          </span>
-          <span className="[grid-area:1/1]">{added ? "Hinzugefügt ✓" : displayLabel}</span>
-        </span>
-      </button>
-      {currentlyOutOfStock && <NotifyMeForm productId={numericId} variantName={variants.length > 0 ? (selectedVariant ?? "") : ""} />}
+        </button>
+      )}
     </div>
   );
 }