fix(admin): stack publish-gallery dialog CTAs so the German label fits (#670)

Reporter @the-luap hit the German `Veröffentlichen & Kunden benachrichtigen`
button overflowing the modal footer in the publish dialog. Two failure modes
chained:

1. The footer was a `flex` row with two `flex-1` buttons inside a
   `max-w-md` (448 px) modal. Default `min-width: auto` on flex children
   meant the primary button kept its content width (~340 px including the
   paper-airplane icon + padding) and pushed the row past the modal frame.

2. Adding `min-w-0 whitespace-normal` doesn't help — the base `.btn` class
   has `@apply ... whitespace-nowrap` (`index.css:149`) which wins over a
   utility className via the Tailwind CSS cascade order. So the text won't
   wrap, the button silently extends past the modal frame, no overflow
   indicator. Verified with `getComputedStyle().whiteSpace = 'normal'` and
   the button still rendering as one ~340 px wide line at ~224 px allocated
   space.

Fix: stack both buttons vertically (`flex flex-col-reverse gap-3`). Primary
appears on top visually (col-reverse), cancel below — standard
confirmation-dialog pattern (Material, Headless UI, Radix all do this for
single-action dialogs). Works in every locale and viewport regardless of
label length. No side-by-side row to overflow.

Tried two prior shapes that didn't hold:
- `flex-col-reverse sm:flex-row` with `sm:flex-1 min-w-0 whitespace-normal`
  on the primary: still overflowed silently because of the
  whitespace-nowrap cascade above.
- `flex-col-reverse sm:flex-row sm:justify-end` with content-width buttons:
  `justify-end` doesn't constrain a row whose content sum exceeds the
  container; row just pushes left of the modal.

Bumping the modal to `max-w-lg` (or wider) was also considered and rejected:
matching modal width is asymmetric (every other admin dialog stays at
`max-w-md`), and any locale longer than German would re-hit the wall.
Stack-always is the only shape that handles every locale + every viewport
without per-language tuning.

Verified end-to-end against a dockerised dev backend:
- DE + EN × desktop (1280px) + mobile (375px) — all four show primary on
  top, cancel below, both inside the modal frame, no overflow.

Lint + tsc + full vitest suite (84/84) clean.

Closes #670.

frontend/src/components/admin/PublishGalleryDialog.tsx

@@ -110,12 +110,21 @@ export const PublishGalleryDialog: React.FC<PublishGalleryDialogProps> = ({
           </div>
         )}
 
-        <div className="flex gap-3">
+        {/* Stack both buttons vertically (always). The German primary label
+            "Veröffentlichen & Kunden benachrichtigen" is ~40 chars including
+            the icon — at max-w-md, no side-by-side row layout fits it on one
+            line, and the base .btn class has @apply whitespace-nowrap (see
+            index.css:149) which overrides a whitespace-normal className via
+            CSS cascade order, so the text won't wrap either. Side-by-side
+            would silently push the button past the modal frame (#670).
+            col-reverse keeps the DOM order semantically secondary-then-primary
+            while putting the primary action visually on top — standard
+            confirmation-dialog pattern. */}
+        <div className="flex flex-col-reverse gap-3">
           <Button
             variant="outline"
             onClick={onClose}
             disabled={isPublishing}
-            className="flex-1"
           >
             {t('common.cancel', 'Cancel')}
           </Button>
@@ -125,7 +134,6 @@ export const PublishGalleryDialog: React.FC<PublishGalleryDialogProps> = ({
             disabled={isPublishing}
             isLoading={isPublishing}
             leftIcon={<Send className="w-4 h-4" />}
-            className="flex-1"
           >
             {t('events.publishAndNotify')}
           </Button>