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/state/index.js

@@ -47,6 +47,7 @@ const DEFAULT_SETTINGS_BASE = Object.freeze({
     display_density: 'default',
     recipes_layout: 'grid',
     card_info_display: 'always',
+    showcase_layout: 'gallery',
     model_name_display: 'model_name',
     lora_syntax_format: 'legacy',
     model_card_footer_action: 'example_images',