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/application-settings-contracts.ts

@@ -1,7 +1,13 @@
 import { z } from 'zod'
 import { magicNoteCommentFormatSchema } from './magic-notes-contracts'
+import { modelDownloadSourceSchema } from './model-download-contracts'
 import { settingsWarningsSchema } from './settings-warning-contracts'
 
+export {
+  modelDownloadSourceSchema,
+  type ModelDownloadSource
+} from './model-download-contracts'
+
 export const magicNoteCommentModeSchema = z.enum([
   'immediate',
   'after-save-auto',
@@ -22,6 +28,7 @@ const applicationPreferencesSchema = z
   .object({
     checkUpdatesOnStartup: z.boolean(),
     updateSource: updateSourceSchema,
+    modelDownloadSource: modelDownloadSourceSchema,
     magicNotesEnabled: z.boolean(),
     magicNoteCommentMode: magicNoteCommentModeSchema,
     magicNoteCommentFormat: magicNoteCommentFormatSchema
@@ -48,6 +55,7 @@ export type ApplicationSettingsUpdate = z.infer<
   typeof applicationSettingsUpdateSchema
 >
 
+
 export type VersionCheckFile = {
   name: string
   size: number