Mastodon, Spotify, Dropbox support #PKCE integration so an browser-only app can get permissions from a user to make API calls on their behalf from a clientside only app. But most other integration possibilities require a server so would cost $ to provide such an integration.
Google & Microsoft appear to have some sort of PKCE support, so it should be possible to do "find friends from google contacts" but app registration looks difficult. At that point, it's easier to ask people to export a csv of their contacts and work with that.
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