Add mention of Synctrain and Möbius Sync (third-party iOS apps) on Downloads page

On the Downloads page, there’s a note under Android pointing users to third-party apps (Syncthing-Fork and Syncthing-Lite), clarifying that they are not official Syncthing releases.

At the moment, there is no equivalent mention for iOS or macOS apps, even though there are active third-party options in the community:

• Synctrain (iOS + macOS) – native client built with SwiftUI, supports selective sync, background syncing, and streaming.
• Möbius Sync (iOS) – sandboxed sync via the iOS Files app.

Since many new users check the Downloads page first, it might be helpful to add a “third-party apps” note for iOS/macOS, modeled after the Android section, e.g.:

### iOS

For iOS, please see the third-party apps [Synctrain](https://apps.apple.com/us/app/synctrain/id6553985316) and [Möbius Sync](https://apps.apple.com/us/app/m%C3%B6bius-sync/id1539203216). Note that they are not official Syncthing releases.

I'll open a PR and edit the downloads.md if you approve.

https://github.com/syncthing/website/blob/main/content%2Fdownloads.md

Start with content/downloads.md and compare the existing Android third-party-app note with the iOS and macOS sections. Add equivalent references to Synctrain and Möbius Sync, clearly stating that they are not official Syncthing releases, then verify both App Store links and the rendered Downloads page.