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/main/index.ts

@@ -440,7 +440,9 @@ if (hasSingleInstanceLock) {
       )
     documentOcrModelManager = new DocumentOcrModelManager({
       userDataDirectory: app.getPath('userData'),
-      fetch: globalThis.fetch
+      fetch: globalThis.fetch,
+      getDownloadSource: async () =>
+        (await applicationSettingsStore.get()).modelDownloadSource
     })
     documentOcrBroker = new DocumentOcrBroker(mainWindow)
     const documentParsingService = new DocumentParsingService(
@@ -456,7 +458,9 @@ if (hasSingleInstanceLock) {
     })
     const speechModelManager = new SpeechModelManager({
       userDataDirectory: app.getPath('userData'),
-      fetch: globalThis.fetch
+      fetch: globalThis.fetch,
+      getDownloadSource: async () =>
+        (await applicationSettingsStore.get()).modelDownloadSource
     })
     const speechTranscriptionService = new SpeechTranscriptionService(
       speechModelManager