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/model-download-targets.ts

@@ -0,0 +1,34 @@
+import {
+  MODEL_DOWNLOAD_REDIRECT_HOSTS,
+  type ModelArtifactTarget
+} from '../shared/model-download-contracts'
+
+export function modelScopeTarget(
+  repository: string,
+  revision: string,
+  file: string
+): ModelArtifactTarget {
+  const repositoryUrl = `https://modelscope.cn/models/${repository}`
+  return {
+    url: `${repositoryUrl}/resolve/${revision}/${file}`,
+    repositoryUrl,
+    revision,
+    redirectHosts: []
+  }
+}
+
+export function huggingFaceTarget(
+  repository: string,
+  revision: string,
+  file: string
+): ModelArtifactTarget {
+  const repositoryUrl = `https://huggingface.co/${repository}`
+  return {
+    url: `${repositoryUrl}/resolve/${revision}/${file}`,
+    repositoryUrl,
+    revision,
+    redirectHosts: [
+      ...MODEL_DOWNLOAD_REDIRECT_HOSTS['hugging-face']
+    ]
+  }
+}