Joining a Fedimint federation is heavy and routinely outlasts the default 15s
client timeout while still succeeding server-side, so the UI wrongly showed
failure. Bump the join timeout to 90s, and on any error re-check the list: if a
new federation appeared the join worked — show 'Federation joined.' instead of
a misleading error.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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