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-TW.json

@@ -525,6 +525,12 @@
                 "hover": "滑鼠懸停顯示"
             },
             "cardInfoDisplayHelp": "選擇何時顯示模型資訊與操作按鈕",
+            "showcaseLayout": "範例圖片版面",
+            "showcaseLayoutHelp": "選擇範例圖片在模型彈窗中的顯示方式：一次一張的圖庫，或傳統的垂直清單。",
+            "showcaseLayoutOptions": {
+                "gallery": "圖庫",
+                "vertical": "垂直清單"
+            },
             "showVersionOnCard": "在卡片上顯示版本",
             "showVersionOnCardHelp": "在模型卡片上顯示或隱藏版本名稱",
             "modelCardFooterAction": "模型卡片按鈕操作",
@@ -1977,7 +1983,9 @@
                 "nsfwMature": "成熟內容",
                 "nsfwR": "R 級內容",
                 "nsfwX": "X 級內容",
-                "nsfwXxx": "XXX 級內容"
+                "nsfwXxx": "XXX 級內容",
+                "layoutGallery": "圖庫檢視",
+                "layoutList": "清單檢視"
             },
             "versions": {
                 "heading": "模型版本",