Show on-device HTML UI mocks as firmware card previews.

Prefer media/preview.html over raster covers so catalog cards mirror each LVGL screen.

Co-authored-by: Cursor <cursoragent@cursor.com>

messages/en.json

@@ -36,7 +36,7 @@
     "noneSelected": "Select a firmware card to see details and flash",
     "selectedLabel": "Selected",
     "docs": "Documentation",
-    "mediaPlaceholder": "Animation / video / cover will appear here"
+    "mediaPlaceholder": "On-device HTML UI preview will appear here"
   },
   "install": {
     "cta": "Connect & flash",

messages/zh.json

@@ -36,7 +36,7 @@
     "noneSelected": "点选固件卡片查看详情并刷写",
     "selectedLabel": "已选",
     "docs": "文档",
-    "mediaPlaceholder": "动画 / 视频 / 封面将显示在这里"
+    "mediaPlaceholder": "设备界面 HTML 预览将显示在这里"
   },
   "install": {
     "cta": "连接并刷写",