Mobile invite sheet: Join retry silently no-ops after claim error

## Summary

After a claim error in the mobile invite join sheet, the error UI re-enables **Join**, but tapping it silently does nothing.

## Root cause

`invite_join_sheet.dart` allows the button in the error state, while the provider's `confirmJoin()` returns early unless status is `confirming`.

## Expected behavior

A retry should either:

- allow `confirmJoin()` from both `confirming` and `error`, or
- keep Join disabled and provide an explicit reset/retry path.

Found during the independent review of PR #1957; non-blocking for that PR but should be fixed as a fast-follow.

Start with invite_join_sheet.dart and the provider's confirmJoin() flow. Reproduce a claim error, then verify that Join either retries successfully from the error state or remains disabled with an explicit reset/retry path; the silent no-op should be gone.

Written by the indexing model from the issue text.