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/zh-CN.json

@@ -525,6 +525,12 @@
                 "hover": "悬停时显示"
             },
             "cardInfoDisplayHelp": "选择何时显示模型信息和操作按钮",
+            "showcaseLayout": "示例图片布局",
+            "showcaseLayoutHelp": "选择示例图片在模型弹窗中的显示方式：一次显示一张的画廊，或经典的纵向列表。",
+            "showcaseLayoutOptions": {
+                "gallery": "画廊",
+                "vertical": "纵向列表"
+            },
             "showVersionOnCard": "在卡片上显示版本",
             "showVersionOnCardHelp": "在模型卡片上显示或隐藏版本名称",
             "modelCardFooterAction": "模型卡片按钮操作",
@@ -1977,7 +1983,9 @@
                 "nsfwMature": "成熟内容",
                 "nsfwR": "R 级内容",
                 "nsfwX": "X 级内容",
-                "nsfwXxx": "XXX 级内容"
+                "nsfwXxx": "XXX 级内容",
+                "layoutGallery": "画廊视图",
+                "layoutList": "列表视图"
             },
             "versions": {
                 "heading": "模型版本",