fix: correct UI copy typos

[holmesworcester]

Summary

• correct acticdvities to activities in the mobile Join Community beta warning
• change the desktop Add Members secondary action from Never mind to Nevermind
• add direct component-test assertions for both corrected strings and update their snapshots

Root cause

Both user-facing strings were hard-coded with the reported copy errors, and the existing tests relied only on snapshots rather than explicit copy assertions.

Testing

• Mobile Join Community focused Jest test: passed (corrected text and snapshot)
• Desktop Add Members focused Jest test: passed (corrected button text and inline snapshot)
• git diff --check: passed

Closes #3380
Closes #3382