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

static/js/managers/SettingsManager.js

@@ -1116,6 +1116,12 @@ export class SettingsManager {
             cardInfoDisplaySelect.value = state.global.settings.card_info_display || 'always';
         }
 
+        // Set showcase layout setting
+        const showcaseLayoutSelect = document.getElementById('showcaseLayout');
+        if (showcaseLayoutSelect) {
+            showcaseLayoutSelect.value = state.global.settings.showcase_layout || 'gallery';
+        }
+
         // Set model card footer action
         const modelCardFooterActionSelect = document.getElementById('modelCardFooterAction');
         if (modelCardFooterActionSelect) {