feat: add AppChooser portal for OpenURI support

[FreddyFunk]

closes #102
also resolves the OpenURI issue mentioned in #3062 when enforcing XWayland.

I tested it with ASHPD Demo and Spotify. I also implement the app chooser backend with a simple UI which allows apps to set a flag to prompt the user with which app they want to open the URI.

2026-03-07.15-37-15.mp4

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