fix(network): explicitly disable proxy fallbacks for none and system modes

Disabled implicit system/environment variable proxy fallbacks in reqwest, yt-dlp, and aria2c when no proxy is explicitly provided. Updated useDownloadStore to strictly return 'none' when 'none' is selected or when 'system' proxy is empty, matching IDM/FDM behavior and ignoring http_proxy environment variables.

src/store/useDownloadStore.ts

@@ -88,15 +88,19 @@ export async function dispatchItem(id: string): Promise<boolean> {
 const getProxyArgs = async (settings: ReturnType<typeof useSettingsStore.getState>) => {
   if (settings.proxyMode === 'system') {
     try {
-      return await invoke('get_system_proxy');
+      const sysProxy = await invoke('get_system_proxy');
+      return typeof sysProxy === 'string' && sysProxy ? sysProxy : "none";
     } catch (e) {
       console.warn("Failed to get system proxy:", e);
-      return null;
+      return "none";
     }
   }
   if (settings.proxyMode === 'custom' && settings.proxyHost) {
     return `http://${settings.proxyHost}:${settings.proxyPort}`;
   }
+  if (settings.proxyMode === 'none') {
+    return "none";
+  }
   return null;
 };