fix(promo-banner): center by default + admin alignment selector (#482)

The gallery promotional banner (#440) read as visually offset from
the gallery footer because:

  - Footer used `container text-center px-4` (full container width,
    centered text).
  - Promo block used `container py-4 sm:py-6` with an inner
    `max-w-3xl mx-auto` wrapper holding left-aligned text — a
    narrower column with left-aligned content sitting in the
    middle of the page.

Two issues compounded: the column was narrower than the footer AND
its text alignment differed. Reported by Rekoo-PS in #482 with a
screenshot showing the misalignment, with a request for an admin
alignment option.

Fix:

- Drop the inner max-w-3xl wrapper. Promo content now spans the
  same .container width as the footer, eliminating the
  narrower-column visual.
- Default text alignment changed from left → center to match the
  footer.
- New `branding_promo_alignment` setting ('left' | 'center' | 'right',
  default 'center'). Surfaced as a dropdown next to the existing
  Position dropdown on the BrandingPage. Live preview block on the
  BrandingPage mirrors the gallery render so admins see what
  guests will see.
- Also replaced the no-op `prose-sm` prose-modifier with a real
  `prose prose-sm` outer class so the existing `prose-a:text-accent`
  modifier actually takes effect (it didn't before — modifiers
  without an outer .prose are silently ignored by Tailwind
  Typography).

Migration 103 seeds the new setting at 'center' so existing
installs that have a promo banner today see the corrected
alignment immediately on next deploy.

i18n: en + de hand-translated; nl/pt/ru/fr machine-translated and
flagged for native review per project convention.

frontend/src/components/gallery/GalleryLayout.tsx

@@ -47,6 +47,9 @@ interface GalleryLayoutProps {
     youtube_url?: string;
     promo_markdown?: string;
     promo_position?: 'above_footer' | 'below_footer';
+    // Horizontal alignment for the promo content (#482). Defaults
+    // to 'center' so the banner aligns with the footer.
+    promo_alignment?: 'left' | 'center' | 'right';
   };
   showLogout?: boolean;
   onLogout?: () => void;
@@ -226,12 +229,36 @@ export const GalleryLayout: React.FC<GalleryLayoutProps> = ({
   })().trim();
   const promoPosition: 'above_footer' | 'below_footer' = brandingSettings?.promo_position === 'below_footer' ? 'below_footer' : 'above_footer';
 
+  // Promo alignment (#482). Defaults to 'center' to match the gallery
+  // footer's `text-center px-4` so the banner reads as part of the
+  // same composition as the footer beneath it. Admin can flip to
+  // 'left' or 'right' from Settings → Branding.
+  const promoAlignment: 'left' | 'center' | 'right' =
+    brandingSettings?.promo_alignment === 'left' ? 'left'
+      : brandingSettings?.promo_alignment === 'right' ? 'right'
+      : 'center';
+  const promoTextAlignClass =
+    promoAlignment === 'left' ? 'text-left'
+      : promoAlignment === 'right' ? 'text-right'
+      : 'text-center';
+
   const promoSlot = promoMarkdown ? (
     <div className="gallery-promo border-t border-surface bg-surface/50">
-      <div className="container py-4 sm:py-6">
-        <div className="max-w-3xl mx-auto text-sm text-theme">
-          <MarkdownContent source={promoMarkdown} className="prose-sm prose-a:text-accent" />
-        </div>
+      {/*
+       * Inner block uses .container (matches the footer's container
+       * width) + the alignment class. We deliberately drop the
+       * previous max-w-3xl wrapper — it created a narrower column
+       * that read as visually offset from the full-width footer
+       * (#482, reported by Rekoo-PS). The `prose` class is needed
+       * for the prose-a:text-accent modifier to actually take effect
+       * (modifiers without an outer .prose are no-ops in Tailwind
+       * Typography).
+       */}
+      <div className={`container py-4 sm:py-6 px-4 ${promoTextAlignClass}`}>
+        <MarkdownContent
+          source={promoMarkdown}
+          className={`prose prose-sm max-w-none mx-auto text-sm text-theme prose-a:text-accent ${promoTextAlignClass}`}
+        />
       </div>
     </div>
   ) : null;

frontend/src/components/gallery/GalleryView.tsx

@@ -253,6 +253,11 @@ export const GalleryView: React.FC<GalleryViewProps> = ({ slug, event }) => {
         youtube_url: settingsData.branding_youtube_url || '',
         promo_markdown: settingsData.branding_promo_markdown || '',
         promo_position: settingsData.branding_promo_position === 'below_footer' ? 'below_footer' : 'above_footer',
+        // Promo alignment (#482). Defaults to 'center' to match the
+        // gallery footer; see GalleryLayout.
+        promo_alignment: ['left', 'center', 'right'].includes(settingsData.branding_promo_alignment)
+          ? settingsData.branding_promo_alignment
+          : 'center',
       });
     }
   }, [settingsData]);