[feat] Add multi-window support for iOS and Android

[velocitysystems]

Is your feature request related to a problem? Please describe.
Multi-window support via the Window API is not currently available on mobile platforms (iOS and Android). This prevents an application from opening, managing and communicating between multiple windows when running on a compatible device i.e. tablet.

Describe the solution you'd like
Implement multi-window support by leveraging native platform capabilities:

• iOS: Starting in iPadOS 13, Apple introduced Scenes allowing apps to support multiple UI instances—each representing its own window or scene. A scene is a separate instance of an app's interface managed by a UIWindowScene and handled via its own SceneDelegate. This has been enhanced further in iPadOS 26 by the new windowing system.
• Android: Starting in Android 12, Google introduced robust support for running multiple app instances (or windows). This has been enhanced further in Android 15 by desktop windowing which allows users to drag UI elements to span new windows and offers system-level window controls.

WRY/TAO layers need to be modified to support: (a) Multiple windows, (b) Multiple webview instances (one per window). Enhancements may need to be made for proper state management. robust memory management and process isolation on both platforms.

Describe alternatives you've considered
Currently no alternatives exist within Tauri. Developers must either:

• Skip multi-window features on mobile
• Build separate native apps
• Accept inconsistent cross-platform behavior

Additional context
For further details see #12140 (Android) and #12142 (iOS).