feat(client): multi-device fan-out + auto-link (v2.2.0-alpha2)

PR #2 of the multi-device roadmap — wires the messenger pipeline
against the on-chain registry landed in v2.2.0-alpha1.

lib/api.ts:
  - DeviceInfo type mirroring blockchain.DeviceInfo.
  - IdentityInfo.device_count (optional; populated from /api/identity).
  - fetchDevices(masterPub) → /api/devices/{master_pub}, returns [].
    Errors swallowed so a downed endpoint doesn't block messaging.
  - resolveRecipientKeys(masterPub) — the routing primitive. Returns
    devices[] if registered, else falls back to IdentityInfo.x25519_pub
    (pre-v2.2.0 path). Empty only when recipient has published nothing.
  - buildLinkDeviceTx / buildUnlinkDeviceTx — signed by master Ed25519,
    min-fee cost, canonical JSON payload matching the chain-side
    LinkDevicePayload / UnlinkDevicePayload.

app/(app)/chats/[id].tsx:
  - sendCore now fans out: encrypts once per recipient device pub
    (Promise.all, any failure rejects the batch), falls back to the
    cached contact.x25519Pub if the registry lookup returns nothing.
  - Saved Messages short-circuit preserved; no devices lookup for self.

app/(app)/_layout.tsx:
  - On every sign-in, auto-submit LINK_DEVICE for this device if its
    X25519 pub isn't already in the master's registry. Device name
    picks "iPhone" / "Android phone" / "Device" by Platform. Errors
    (insufficient balance / legacy chain without LINK_DEVICE support)
    are silent — next launch retries.

Backward compatibility: senders fall back to identity.x25519_pub when
the recipient has no registry entries, so pre-v2.2.0 clients still
receive messages. Chain-side already gates new validation on the event
types existing; old clients simply never emit LINK_DEVICE and keep
working with a single X25519.

Next — PR #3 (Settings → Devices screen + QR pairing flow + receive-side
self-wipe on revoke detection).

client-app/app/(app)/_layout.tsx

@@ -13,7 +13,7 @@
  * один раз; переходы между tab'ами их не перезапускают.
  */
 import React, { useEffect } from 'react';
-import { View } from 'react-native';
+import { View, Platform } from 'react-native';
 import { router, usePathname } from 'expo-router';
 import { useSafeAreaInsets } from 'react-native-safe-area-context';
 import { useStore } from '@/lib/store';
@@ -26,6 +26,9 @@ import { getWSClient } from '@/lib/ws';
 import { NavBar } from '@/components/NavBar';
 import { AnimatedSlot } from '@/components/AnimatedSlot';
 import { saveContact } from '@/lib/storage';
+import {
+  fetchDevices, buildLinkDeviceTx, submitTx,
+} from '@/lib/api';
 
 export default function AppLayout() {
   const keyFile  = useStore(s => s.keyFile);
@@ -73,6 +76,44 @@ export default function AppLayout() {
     else ws.setAuthCreds(null);
   }, [keyFile]);
 
+  // Multi-device registry bootstrap (v2.2.0). On every sign-in:
+  //   1. Fetch our own device list from chain.
+  //   2. If our local X25519 pub isn't in the active set, submit a
+  //      LINK_DEVICE tx for it — this makes "this device" discoverable
+  //      to senders. No-ops after the first successful submission on
+  //      a given chain + master key pair.
+  // Failures are swallowed: insufficient balance, offline node, or a
+  // chain that hasn't upgraded to v2.2.0 all surface the same way
+  // (our device just isn't registered yet; next sign-in tries again).
+  useEffect(() => {
+    if (!keyFile) return;
+    let cancelled = false;
+    (async () => {
+      try {
+        const devs = await fetchDevices(keyFile.pub_key);
+        if (cancelled) return;
+        if (devs.some(d => d.x25519_pub_key === keyFile.x25519_pub)) {
+          return; // already registered
+        }
+        const deviceName = Platform.select({
+          ios:     'iPhone',
+          android: 'Android phone',
+          default: 'Device',
+        }) ?? 'Device';
+        const tx = buildLinkDeviceTx({
+          from:       keyFile.pub_key,
+          x25519Pub:  keyFile.x25519_pub,
+          deviceName,
+          privKey:    keyFile.priv_key,
+        });
+        await submitTx(tx);
+      } catch {
+        /* best-effort — next launch retries */
+      }
+    })();
+    return () => { cancelled = true; };
+  }, [keyFile]);
+
   useEffect(() => {
     if (keyFile === null) {
       const t = setTimeout(() => {