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/migrations/core/103_add_promo_alignment_setting.js

@@ -0,0 +1,43 @@
+/**
+ * Migration: Promotional banner text alignment (#482).
+ *
+ * Adds `branding_promo_alignment` to app_settings — admin-controlled
+ * horizontal alignment for the gallery promotional banner content
+ * (#440 / #482). Reuses the existing app_settings shape that
+ * branding_promo_markdown / branding_promo_position already use.
+ *
+ * Default 'center' so the banner aligns with the gallery footer
+ * (which is full-width center-aligned). The previous default left
+ * the markdown left-aligned in a max-w-3xl block, which Rekoo-PS
+ * reported as visually offset from the footer.
+ *
+ * Allowed values: 'left' | 'center' | 'right' — validated on the
+ * write path in adminSettings.js, not enforced by the column type
+ * (we use varchar instead of CHECK so the value can be extended
+ * later — e.g. 'justify' — without another schema migration).
+ *
+ * Idempotent: skips the insert when the row already exists.
+ */
+
+exports.up = async function(knex) {
+  if (!(await knex.schema.hasTable('app_settings'))) return;
+
+  const existing = await knex('app_settings')
+    .where('setting_key', 'branding_promo_alignment')
+    .first();
+  if (existing) return;
+
+  await knex('app_settings').insert({
+    setting_key: 'branding_promo_alignment',
+    setting_value: JSON.stringify('center'),
+    setting_type: 'branding',
+    updated_at: new Date(),
+  });
+};
+
+exports.down = async function(knex) {
+  if (!(await knex.schema.hasTable('app_settings'))) return;
+  await knex('app_settings')
+    .where('setting_key', 'branding_promo_alignment')
+    .del();
+};