### What changed?
After a new mobile invite claim succeeds, Buzz now best-effort ensures
membership in the same public starter channels as desktop:
- `#general`
- `#welcome-everyone`
Missing starters use desktop's deterministic per-relay IDs and exact
public channel configuration, so concurrent mobile and desktop setup
converges safely. Setup failures do not invalidate or retry an already
successful invite claim, and failure for one starter does not block the
other.
The success sheet offers **Continue to #welcome-everyone** when that
channel is available. Mobile does not create the private `Welcome`
channel because it cannot provision the desktop Welcome agents that make
that channel useful.
This PR is stacked on #6145 because it deliberately reuses that PR's
open-channel directory and join behavior. Once #6145 merges, this PR can
be retargeted to `main` without changing its BUZZ-12 diff.
Fixes
[BUZZ-12](https://linear.app/squareup/issue/BUZZ-12/bug-community-appears-empty-after-using-invite-link-on-mobile).
### How is it tested?
- Desktop/mobile deterministic starter-ID parity coverage.
- Existing-channel join and missing-channel creation coverage.
- Duplicate-create convergence and per-channel failure isolation
coverage.
- Invite success remains successful when starter setup fails.
- Widget coverage for continuing directly into `#welcome-everyone`.
- Focused invite/deep-link tests: 23 passed.
- `just mobile-check`: passed.
- `just mobile-test`: 1,483 passed.
- Pre-push repository checks: passed.
---------
Signed-off-by: Tom Brow <tomb@block.xyz>
Co-authored-by: Codex <noreply@openai.com>
