[feat] iOS: theme resolution / subscription

[luixo]

Describe the problem

Currently, there is no way to get current theme (color mode) on iOS.
Webview object doesn't have the same methods as desktop (and I'm not sure it react on tauri://theme-changed event, couldn't reproduce on simulator) e.g. onThemeChanged() and theme().

Describe the solution you'd like

Window should not throw Window does not exist on Mobile client / Webview should offer theme-related methods.

Alternatives considered

No "system" theme on iOS.

Additional context

No response