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.

backend/src/routes/adminSettings.js

@@ -322,7 +322,8 @@ router.put('/branding', adminAuth, requirePermission('settings.edit'), async (re
       twitter_url,
       youtube_url,
       promo_markdown,
-      promo_position
+      promo_position,
+      promo_alignment
     } = req.body;
 
     // Normalize force_color_mode: only 'dark' | 'light' | null are valid.
@@ -340,6 +341,15 @@ router.put('/branding', adminAuth, requirePermission('settings.edit'), async (re
       ? 'below_footer'
       : 'above_footer';
 
+    // Normalize promo_alignment: 'left' | 'center' | 'right'. Defaults
+    // to 'center' to match the gallery footer's full-width centering
+    // (#482 — the previous default left the markdown left-aligned in
+    // a max-w-3xl block, which read as visually offset from the footer).
+    const allowedPromoAlignments = ['left', 'center', 'right'];
+    const normalizedPromoAlignment = allowedPromoAlignments.includes(promo_alignment)
+      ? promo_alignment
+      : 'center';
+
     // Normalize login_logo_size to the same token set as logo_size.
     // Anything else falls back to 'medium' on the next render.
     const allowedLoginLogoSizes = ['small', 'medium', 'large', 'xlarge'];
@@ -381,7 +391,8 @@ router.put('/branding', adminAuth, requirePermission('settings.edit'), async (re
       ...(twitter_url !== undefined   && { twitter_url:   String(twitter_url   || '').trim() }),
       ...(youtube_url !== undefined   && { youtube_url:   String(youtube_url   || '').trim() }),
       ...(promo_markdown !== undefined && { promo_markdown: typeof promo_markdown === 'string' ? promo_markdown : '' }),
-      ...(promo_position !== undefined && { promo_position: normalizedPromoPosition })
+      ...(promo_position !== undefined && { promo_position: normalizedPromoPosition }),
+      ...(promo_alignment !== undefined && { promo_alignment: normalizedPromoAlignment })
     };
 
     // Handle favicon deletion if empty string or null is provided

backend/src/routes/publicSettings.js

@@ -77,6 +77,12 @@ router.get('/', async (req, res) => {
       branding_promo_position: settingsObject.branding_promo_position === 'below_footer'
         ? 'below_footer'
         : 'above_footer',
+      // Promo content alignment (#482). Defaults to center so the
+      // banner aligns with the gallery footer; admin can flip to
+      // left or right via Settings → Branding.
+      branding_promo_alignment: ['left', 'center', 'right'].includes(settingsObject.branding_promo_alignment)
+        ? settingsObject.branding_promo_alignment
+        : 'center',
       // Force a specific color mode site-wide. When set, the user toggle
       // is hidden and the value overrides per-theme/system preference.
       // Allowed values: 'dark' | 'light' | null (null = no force).