fix: pass participant numbers when relaunching new group thread

[mvanhorn]

Summary

Creating a group message no longer dead-ends with "An unknown error occurred" on devices whose telephony provider can't resolve participants for a freshly created thread id (the MIUI/Xiaomi failure in the report). When the add-contacts flow relaunches ThreadActivity with the new group's thread id, the intent now also carries the participant numbers (putExtra(THREAD_NUMBER, Gson().toJson(numbers))), and the empty-participants fallback in setupThread builds one participant per number via the existing getPhoneNumbersFromIntent() helper instead of reading a single plain-string extra.

Why this matters

The reporter in #41 could not create any group message: the compose flow computed the new thread id, relaunched the thread screen with only THREAD_ID, MIUI's provider returned no participants for that id, and the fallback found no THREAD_NUMBER extra so it toasted the unknown-error message and closed the screen. The helper this fix routes through already parses both a plain number and a Gson JSON array, so single-recipient relaunches behave exactly as before; the toast-and-finish path still fires when no numbers are available at all.

Testing

Traced both relaunch paths: existing single-number intents take the same one-participant branch as before, and the new group path round-trips the numbers through the JSON array format getPhoneNumbersFromIntent() already supports (ThreadActivity.kt:1798). Added the CHANGELOG "Fixed" entry per repo convention.

Fixes #41