Join-via-link handler always returns None — join links non-functional

[intendednull]

Problem

The JoinRequest handler in crates/client/src/listeners.rs:309-339 validates join links and increments the used counter, but the invite generation closure always returns None::<String> with a comment "For now, just return None — full invite generation needs refactoring."

This means:

• Valid join requests are silently dropped after consuming a use count
• The link's used counter is incremented even though no invite is sent
• Join-via-link is completely non-functional

Suggested fix

Implement the invite generation in the JoinRequest handler, or stop incrementing the used counter until invite generation is implemented.

Location

crates/client/src/listeners.rs:309-339

Found during audit of #26.