feat(ui): add optional vertical list layout for model modal showcase

Bring back the pre-v1.2.2 classic vertical list for example images as a
persisted showcase_layout setting (gallery stays the default), switchable
from both a Settings select and an in-modal segmented toggle.

- Revive the vertical list renderer (adapted from 4a6042d0) as
  VerticalListView.js, reusing the shared MediaUtils/MetadataPanel
  infrastructure; legacy CSS scoped under .showcase-vertical
- Dispatch showcase rendering on state.settings.showcase_layout;
  collapsed bar, empty/filtered states and import flow stay shared
- Keep the back-to-top button available in vertical mode (the
  showcase-expanded class that hides it now only applies to the gallery,
  whose thumbnail strip occupies that corner)
- Persist the setting via DEFAULT_SETTINGS + DEFAULT_SETTINGS_BASE and a
  new select under Settings > Layout Settings
- i18n: 6 new keys translated in all 9 locales; terminology recorded in
  docs/i18n-translation-guidelines.md

Closes #1136

templates/components/modals/settings/interface.html

@@ -87,6 +87,11 @@
             ('hover', 'settings.layoutSettings.cardInfoDisplayOptions.hover'),
         ], 'settings.layoutSettings.cardInfoDisplayHelp') }}
 
+        {{ sm.setting_select('showcaseLayout', 'showcase_layout', 'settings.layoutSettings.showcaseLayout', [
+            ('gallery', 'settings.layoutSettings.showcaseLayoutOptions.gallery'),
+            ('vertical', 'settings.layoutSettings.showcaseLayoutOptions.vertical'),
+        ], 'settings.layoutSettings.showcaseLayoutHelp') }}
+
         <div class="setting-item">
             <div class="setting-row">
                 <div class="setting-info">