feat: add custom download URL for manual update method

- Add custom_download_url field to Version model
- Show custom download URL input when update method is 'manual'
- Use platform default URL if custom URL is not set
- Add i18n translations for new field

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/plugins/i18n/en.json

@@ -1499,6 +1499,11 @@
         "previewUpdateType": "Update Type",
         "previewUpdateMethod": "Update Method",
         "previewAutoUpdate": "Auto Update",
+        "customDownloadUrl": "Custom Download URL",
+        "customDownloadUrlDesc": "Set a custom download URL, or use the platform default if not set",
+        "customDownloadUrlPlaceholder": "e.g., https://your-cdn.com/download/app.zip",
+        "previewCustomDownloadUrl": "Download URL",
+        "useDefault": "Use Default",
         "changelogPreview": "Changelog Preview",
         "noChangelog": "No changelog",
         "notUploaded": "Not uploaded",

frontend/src/plugins/i18n/zh.json

@@ -1477,6 +1477,11 @@
         "previewUpdateType": "更新类型",
         "previewUpdateMethod": "更新方式",
         "previewAutoUpdate": "自动更新",
+        "customDownloadUrl": "自定义下载地址",
+        "customDownloadUrlDesc": "设置自定义下载地址，不设置则使用平台默认地址",
+        "customDownloadUrlPlaceholder": "例如: https://your-cdn.com/download/app.zip",
+        "previewCustomDownloadUrl": "下载地址",
+        "useDefault": "使用默认地址",
         "changelogPreview": "更新日志预览",
         "noChangelog": "暂无更新日志",
         "notUploaded": "未上传",