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/services/publicSettings.service.ts

@@ -39,6 +39,9 @@ export interface PublicSettings {
   branding_youtube_url?: string;
   branding_promo_markdown?: string;
   branding_promo_position?: 'above_footer' | 'below_footer';
+  // Per-install promo banner alignment (#482). Defaults to 'center'
+  // so the banner aligns with the gallery footer's centering.
+  branding_promo_alignment?: 'left' | 'center' | 'right';
   theme_config: any;
   default_language: string;
   enable_analytics: boolean;

frontend/src/services/settings.service.ts

@@ -42,6 +42,9 @@ export interface BrandingSettings {
   youtube_url?: string;
   promo_markdown?: string;
   promo_position?: 'above_footer' | 'below_footer';
+  // Per-install promo banner alignment (#482). Defaults to center
+  // so the banner aligns with the gallery footer.
+  promo_alignment?: 'left' | 'center' | 'right';
 }
 
 export interface ThemeSettings {
@@ -336,7 +339,10 @@ export const settingsService = {
       twitter_url: rawSettings.branding_twitter_url || '',
       youtube_url: rawSettings.branding_youtube_url || '',
       promo_markdown: rawSettings.branding_promo_markdown || '',
-      promo_position: rawSettings.branding_promo_position === 'below_footer' ? 'below_footer' : 'above_footer'
+      promo_position: rawSettings.branding_promo_position === 'below_footer' ? 'below_footer' : 'above_footer',
+      promo_alignment: ['left', 'center', 'right'].includes(rawSettings.branding_promo_alignment)
+        ? rawSettings.branding_promo_alignment
+        : 'center'
     };
   },