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

locales/en.json

@@ -525,6 +525,12 @@
         "hover": "Reveal on Hover"
       },
       "cardInfoDisplayHelp": "Choose when to display model information and action buttons",
+      "showcaseLayout": "Showcase Layout",
+      "showcaseLayoutHelp": "Choose how example images are displayed in the model modal: a one-at-a-time gallery or the classic vertical list.",
+      "showcaseLayoutOptions": {
+        "gallery": "Gallery",
+        "vertical": "Vertical List"
+      },
       "showVersionOnCard": "Show Version on Card",
       "showVersionOnCardHelp": "Show or hide the version name on model cards",
       "modelCardFooterAction": "Model Card Button Action",
@@ -1977,7 +1983,9 @@
         "nsfwMature": "Mature Content",
         "nsfwR": "R-rated Content",
         "nsfwX": "X-rated Content",
-        "nsfwXxx": "XXX-rated Content"
+        "nsfwXxx": "XXX-rated Content",
+        "layoutGallery": "Gallery view",
+        "layoutList": "List view"
       },
       "versions": {
         "heading": "Model versions",