fix(mesh): message federation contacts without a radio (fixes 'Missing contact_id')

Messaging a federation-only peer (e.g. 'Arch Dev') failed with 'Missing
contact_id'. The UI gave federation-only rows a *negative* placeholder
contact_id derived from a DID hash, but the backend parses contact_id as u64,
so a negative value deserialized to None. The negative id also never matched
the positive federation-synthetic id that federation-routed messages are stored
under, so those threads looked empty.

- Frontend: derive the SAME positive federation-synthetic id the backend uses
  (federationContactId mirrors federation_peer_contact_id) so mesh.send accepts
  it and messages thread correctly.
- Backend: send_typed_wire now resolves a federation-synthetic contact_id from
  nodes.json when it isn't in the live mesh peer table (radio-less node),
  instead of bailing 'Unknown federation peer'.

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

core/archipelago/src/mesh/mod.rs

@@ -825,15 +825,33 @@ impl MeshService {
         let is_federation_synthetic = contact_id & 0x8000_0000 != 0;
         let exceeds_lora = wire.len() > protocol::MAX_MESSAGE_LEN;
         if is_federation_synthetic || exceeds_lora {
-            let (peer_pubkey, peer_did) = {
+            // Resolve the peer's pubkey/did. Prefer the live mesh peer table,
+            // but fall back to federation storage for federation-synthetic ids
+            // that were never seeded into `state.peers` — e.g. a radio-less
+            // node where the mesh device table is empty. Without this fallback
+            // chatting a federation contact bails "Unknown federation peer"
+            // even though we know its onion from nodes.json.
+            let from_table = {
                 let peers = self.state.peers.read().await;
-                match peers.get(&contact_id) {
-                    Some(p) => (p.pubkey_hex.clone(), p.did.clone()),
-                    None if is_federation_synthetic => {
-                        anyhow::bail!("Unknown federation peer {}", contact_id);
+                peers
+                    .get(&contact_id)
+                    .map(|p| (p.pubkey_hex.clone(), p.did.clone()))
+            };
+            let (peer_pubkey, peer_did) = match from_table {
+                Some(v) => v,
+                None if is_federation_synthetic => {
+                    let nodes = crate::federation::load_nodes(&self.data_dir)
+                        .await
+                        .unwrap_or_default();
+                    match nodes
+                        .iter()
+                        .find(|n| federation_peer_contact_id(&n.pubkey) == contact_id)
+                    {
+                        Some(n) => (Some(n.pubkey.clone()), Some(n.did.clone())),
+                        None => anyhow::bail!("Unknown federation peer {}", contact_id),
                     }
-                    None => (None, None),
                 }
+                None => (None, None),
             };
             let nodes = crate::federation::load_nodes(&self.data_dir)
                 .await

neode-ui/src/views/Mesh.vue

@@ -626,10 +626,12 @@ const mergedPeers = computed<MergedPeer[]>(() => {
     if (groups.has(key)) continue
     // Synthesise a placeholder MeshPeer so openChat() and the existing
     // rssi/avatar template paths don't need a separate code path for
-    // "federation-only" rows. The contact_id is a stable negative number
-    // derived from the DID hash so it never collides with real mesh
-    // contact_ids (which are u32 from the radio firmware).
-    const synthCid = -100 - Math.abs(hashStringToInt(fed.did))
+    // "federation-only" rows. The contact_id MUST match the backend's
+    // federation-synthetic id (high bit set, derived from the pubkey) so
+    // `mesh.send` accepts it (it parses contact_id as u64 — a negative
+    // placeholder fails with "Missing contact_id") and so federation-routed
+    // messages stored under that id land in this thread.
+    const synthCid = federationContactId(fed.pubkey)
     const placeholder: MeshPeer = {
       contact_id: synthCid,
       advert_name: fed.name || fed.did,
@@ -657,10 +659,16 @@ const mergedPeers = computed<MergedPeer[]>(() => {
   return Array.from(groups.values())
 })
 
-function hashStringToInt(s: string): number {
-  let h = 0
-  for (let i = 0; i < s.length; i++) h = (h * 31 + s.charCodeAt(i)) | 0
-  return h
+// Mirror of the backend's `federation_peer_contact_id` (mesh/mod.rs): take the
+// first 4 bytes of the archipelago pubkey as a little-endian u32, clear the top
+// bit, then set it as the federation marker. Producing the SAME id here means a
+// federation-only row addresses the exact contact the backend routes/stores
+// under, instead of an unsendable negative placeholder.
+function federationContactId(pubkeyHex: string): number {
+  const bytes = pubkeyHex.match(/../g)?.map(h => parseInt(h, 16)) ?? []
+  if (bytes.length < 4) return 0x80000001
+  const low = ((bytes[0]) | (bytes[1] << 8) | (bytes[2] << 16) | (bytes[3] << 24)) >>> 0
+  return ((0x80000000 | (low & 0x7fffffff)) >>> 0)
 }
 
 // activeChatPeer is a single MeshPeer (the row the user clicked). To unify