feat: desktop messaging + pairing + cross-client master-pub attribution (v2.2.0-alpha5)

Two coordinated changes:

1. Desktop client gets a functional Messages section and working pairing
flow, putting it at feature parity with mobile for the v2.2.0 line.

2. Server + both clients teach each other to use the sender's master
Ed25519 (not just their X25519) to address conversations, so a peer
writing from a different linked device still rolls into the same chat.
This is the "new API logic" the desktop scaffold was waiting on.

Server (node/api_relay.go, cmd/node/main.go):
  * /relay/inbox items now carry `sender_ed25519_pub` alongside the
    per-device `sender_pub`. Empty string for pre-v2.2.0 senders.
  * WS `inbox` push summary also includes `sender_ed25519_pub`, so the
    client can skip the refetch when the envelope plainly isn't for
    the chat they're watching.
  * Both existing tests pass.

Mobile client:
  * lib/types.ts Envelope grew `sender_ed25519_pub`; fetchInbox normalises
    it (default '') for older nodes.
  * hooks/useGlobalInbox matches contacts by (master Ed25519 OR legacy
    X25519) so an incoming message from a peer's desktop reuses the
    existing chat instead of creating a duplicate placeholder.
  * hooks/useMessages now takes an optional `contactMasterEd25519` and
    exposes a matchesChat() predicate; WS inbox handler uses it too to
    avoid spurious refetches.
  * chats/[id].tsx passes `contact.address` (master) along with x25519.

Desktop client — all new:
  * src/lib/crypto.ts — tweetnacl hex/base64 helpers, generateKeyFile,
    encryptMessage/decryptMessage, signBase64, shortAddr. Same signatures
    as the mobile lib; uses Chromium's window.crypto, no expo-crypto dep.
  * src/lib/tx.ts — buildTransferTx / buildLinkDeviceTx / buildUnlinkDeviceTx
    + submitTx + humanizeTxError, canonical-bytes identical to mobile.
  * src/lib/relay.ts — fetchInbox, sendEnvelope, resolveRecipientKeys
    (multi-device fan-out with legacy identity.x25519 fallback).
  * src/lib/store.ts — zustand state gets messages{}, unread{},
    activeChat.
  * src/lib/storage.ts — per-chat cache via localStorage (500-msg cap).
  * src/hooks/useInboxPoll — 4s polling loop, addresses conversations
    by master Ed25519, bumps unread unless chat is active.
  * src/sections/messages/* — ChatList (sorted tiles, unread badges),
    Conversation (auto-scroll messages + composer + fan-out send,
    Enter-to-send / Shift+Enter for newline), EmptyConversation.
  * src/auth/Pair.tsx — 6-digit code + device key screen, polls inbox
    for a handshake envelope, assembles the KeyFile on arrival.
  * Welcome.tsx: Pair button now actually routes to <Pair>; imports
    generateKeyFile from lib/crypto (was inlined).

docs/ROADMAP.md delta: alpha5 row flipped to done inline. Alpha6
(feed + wallet) and rc1 (contacts + devices UI + profile) still
pending.

cmd/node/main.go

@@ -684,11 +684,17 @@ func main() {
 	// /relay/inbox if it needs the full envelope. Keeps WS frames small and
 	// avoids a fat push for every message.
 	mailbox.SetOnStore(func(env *relay.Envelope) {
+		// Summary only — no ciphertext. Multi-device (v2.2.0+) clients
+		// use sender_ed25519_pub to decide whether the envelope belongs
+		// to the chat they're currently viewing (messages from any of
+		// the peer's linked devices share a master identity), so the
+		// field must be in every push.
 		sum, _ := json.Marshal(map[string]any{
-			"id":            env.ID,
-			"recipient_pub": env.RecipientPub,
-			"sender_pub":    env.SenderPub,
-			"sent_at":       env.SentAt,
+			"id":                 env.ID,
+			"recipient_pub":      env.RecipientPub,
+			"sender_pub":         env.SenderPub,
+			"sender_ed25519_pub": env.SenderEd25519PubKey,
+			"sent_at":            env.SentAt,
 		})
 		eventBus.EmitInbox(env.RecipientPub, sum)
 	})