feat: add explicit managed model download sources

Managed speech and OCR downloads previously used catalog-specific source URLs without a global selection. Platform Features now lets users choose ModelScope by default or Hugging Face, while Main validates and freezes that source for each download.

Verified coverage remains explicit: downloads never mix artifacts or silently switch sources, and installed models plus ZIP imports stay source-independent.

Release note: 可在“设置 → 平台功能 → 通用设置”中选择 ModelScope 或 Hugging Face 作为后续语音输入与 OCR 模型下载源；缺少完整已验证文件或下载失败时不会静默换源。

src/shared/goodbuddy-config-contracts.test.ts

@@ -134,6 +134,7 @@ describe('GoodBuddy configuration contracts', () => {
       application: {
         checkUpdatesOnStartup: true,
         updateSource: 'github',
+        modelDownloadSource: 'modelscope',
         magicNotesEnabled: true,
         magicNoteCommentMode: 'immediate',
         magicNoteCommentFormat: 'combined'