feat(client): device pairing flow (v2.2.0-alpha3)

Completes PR #3 of the multi-device roadmap. Two devices of the same
identity can now be linked via a six-digit code + relay envelope
handshake. Chain-level fan-out (alpha2) and self-wipe on revoke (this
release, earlier commit) already work end-to-end.

New device — app/(auth)/pair.tsx:
  * Generates fresh X25519 keypair + six-digit code locally.
  * Displays them for transcription on the primary device.
  * Polls /relay/inbox on its own x25519 pub every 2.5s.
  * Decrypts each envelope with the session priv + sender_pub.
  * Accepts only payloads where {v=1, type=pair-handshake, code matches}.
  * On success, assembles a KeyFile (master Ed25519 from envelope,
    x25519 from session) and redirects into (app).

Primary device — app/(app)/devices.tsx:
  * "Link new device" opens a modal asking for {code, device key, name}.
  * On submit: builds+submits LINK_DEVICE tx for the new pub, then
    sends a one-shot relay envelope carrying {master_pub, master_priv,
    master_x25519_pub, code} encrypted for the new device's x25519 pub.
  * Optimistic local insert so the new row appears immediately.

Entry point — app/index.tsx:
  * Third button on welcome slide 3: "Pair". Routes to /auth/pair.
    Create / Import remain unchanged for fresh identities.

Security:
  * Master Ed25519 priv leaves the source device ONLY inside an envelope
    sealed with NaCl box for the new device's X25519 pub. The relay node
    sees only ciphertext.
  * Six-digit code (~20 bits) gates acceptance — an attacker who guesses
    both a session pub AND the code is still filtered by the X25519
    decryption itself (code match is belt-and-suspenders).
  * Envelope stays in relay mailbox until TTL — no DELETE call yet;
    idempotent on our side (saveKeyFile overwrites, session pub
    never polled after redirect).

Known trade-offs:
  * Manual transcription of a 64-char hex key is ugly. Alpha4 will
    offer a QR fallback on phones with cameras; desktop keeps typing.
  * No rate limit on the polling. Fine for a 1-minute handshake, needs
    cap-on-stale if a user leaves the screen open.

client-app/app/index.tsx

@@ -346,8 +346,13 @@ export default function WelcomeScreen() {
           {/* CTA — прижата к правому нижнему краю. */}
           <View style={{
             flexDirection: 'row', justifyContent: 'flex-end', gap: 10,
-            paddingHorizontal: 24, paddingBottom: 8,
+            paddingHorizontal: 24, paddingBottom: 8, flexWrap: 'wrap',
           }}>
+            <CTASecondary
+              label="Pair"
+              icon="link"
+              onPress={() => router.push('/(auth)/pair' as never)}
+            />
             <CTASecondary
               label="Import"
               onPress={() => router.push('/(auth)/import' as never)}