fix: multi-hop federation — hub relay forwards without local participants

Three fixes for 3-relay chain (R1→R2→R3):

1. Room lookup in handle_datagram: hub relay (R2) has no local
   participants, so active_rooms() was empty and datagrams were
   silently dropped. Now also checks global_rooms config directly,
   allowing hub relays to forward without local clients.

2. Multi-hop forwarding: removed active_rooms filter — forward to
   ALL connected peers except source. The receiving peer decides
   whether to deliver or forward further.

3. Android relay_label: native RoomMember now includes relay_label
   from RoomUpdate signal. Kotlin UI reads it for relay grouping.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-android/src/stats.rs

@@ -76,4 +76,5 @@ pub struct CallStats {
 pub struct RoomMember {
     pub fingerprint: String,
     pub alias: Option<String>,
+    pub relay_label: Option<String>,
 }