fix(mesh): collapse cross-transport twin contacts into one conversation (#12)

A node reachable both over LoRa and federation has two MeshPeer rows (radio
twin: low contact_id + firmware key; federation twin: high contact_id +
archipelago key), and messages key by peer_contact_id split across the two ids
— so opening one twin shows an empty thread (the .120->.89 symptom).

- backend: new group_peer_twins() helper groups peers by arch_pubkey_hex (set on
  BOTH twins by bind_federation_twins), keeps the radio id as the mesh-first
  send target, and unions messages across all twin ids. Wired into
  conversations.list / conversations.messages / mesh.contacts-list. +3 unit tests.
- frontend: the live chat list merges client-side (mergedPeers) and matched twins
  by the "Archy-z6Mk..." advert prefix, which the Meshtastic device rename broke
  (radio now advertises the server name). Merge by arch_pubkey_hex instead, which
  the backend reliably sets on both twins. Expose arch_pubkey_hex on MeshPeer.
- fix unrelated stale test: EcashTransaction test missing the new `kind` field.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/archipelago/src/wallet/ecash.rs

@@ -1507,6 +1507,7 @@ mod tests {
             description: "test".into(),
             mint_url: String::new(),
             peer: String::new(),
+            kind: default_tx_kind(),
         };
         let json = serde_json::to_string(&tx).unwrap();
         assert!(json.contains("\"streamingpayment\""));