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-transport.ts

@@ -0,0 +1,71 @@
+const MAX_REDIRECTS = 3
+const redirectStatuses = new Set([301, 302, 303, 307, 308])
+
+function validateDownloadUrl(value: string, modelLabel: string): URL {
+  const url = new URL(value)
+  if (
+    url.protocol !== 'https:' ||
+    (url.port !== '' && url.port !== '443') ||
+    url.username ||
+    url.password ||
+    url.hash
+  ) {
+    throw new Error(
+      `${modelLabel}下载地址必须是使用标准端口、无凭据和 Fragment 的 HTTPS URL`
+    )
+  }
+  return url
+}
+
+export async function fetchModelDownloadResponse(options: {
+  transport: typeof fetch
+  initialUrl: string
+  redirectHosts: readonly string[]
+  signal: AbortSignal
+  modelLabel: string
+}): Promise<Response> {
+  let url = validateDownloadUrl(options.initialUrl, options.modelLabel)
+  const initialHost = url.hostname
+  const allowedRedirectHosts = new Set(options.redirectHosts)
+  for (let redirectCount = 0; ; redirectCount += 1) {
+    if (options.signal.aborted) {
+      throw new DOMException('The operation was aborted', 'AbortError')
+    }
+    const response = await options.transport(url, {
+      method: 'GET',
+      redirect: 'manual',
+      credentials: 'omit',
+      cache: 'no-store',
+      signal: options.signal
+    })
+    if (!redirectStatuses.has(response.status)) {
+      return response
+    }
+    if (redirectCount >= MAX_REDIRECTS) {
+      await response.body?.cancel().catch(() => undefined)
+      throw new Error(
+        `${options.modelLabel}下载重定向次数过多`
+      )
+    }
+    const location = response.headers.get('location')
+    await response.body?.cancel().catch(() => undefined)
+    if (!location) {
+      throw new Error(
+        `${options.modelLabel}下载重定向缺少地址`
+      )
+    }
+    const nextUrl = validateDownloadUrl(
+      new URL(location, url).toString(),
+      options.modelLabel
+    )
+    if (
+      nextUrl.hostname !== initialHost &&
+      !allowedRedirectHosts.has(nextUrl.hostname)
+    ) {
+      throw new Error(
+        `${options.modelLabel}下载重定向到未声明的主机`
+      )
+    }
+    url = nextUrl
+  }
+}