feat(chat): composer + contact requests — outgoing construction (Phase 4)

Add the outgoing side of HushChat: compose messages and start conversations.
The wire construction is the byte-exact inverse of the receive parser and is
proven by a self-consistent round-trip (build → parse → decrypt).

- src/chat/chat_outgoing.{h,cpp} (pure): buildOutgoingMessage encrypts via
  encryptOutgoing and buildOutgoingContactRequest carries plaintext; both emit
  the header memo JSON ({h,v,z,cid,t,e,p}, which nlohmann serializes
  alphabetically to match SilentDragonXLite) + the payload memo, validating the
  512-byte memo limit, the 64-hex peer key, and the "no leading '{'" rule for
  request text. My public key goes in header "p"; the peer's key is the
  encryption recipient.
- ChatService: composeMessage/composeContactRequest (encrypt with the held
  identity) + recordOutgoing (echo an Outgoing ChatMessage into the store + DB —
  we never harvest our own sends, so this is the only local record).
- App: sendChatMessage(cid,text) sources the peer z-addr + public key from the
  conversation, composes, and records a random-id echo; startChatConversation
  (zaddr,text) mints a random cid + composes a plaintext contact request;
  chatReplyZaddr() picks a spendable z-addr. broadcastChatMemos() is the Phase-5
  transport seam (network delivery + real-SDXL interop verification land there).
- Chat tab: a message composer (shown once the peer's key is known, else a
  "waiting for reply" hint) + a "New conversation" modal that sends a contact
  request to a z-address.
- Tests: outgoing round-trip through the receive parser + decrypt, contact-request
  passthrough, validation guards, and ChatService compose + recordOutgoing echo.

Adversarially reviewed (crypto/interop, secret hygiene, logic, UI) — 0 findings.
Gated by DRAGONX_ENABLE_CHAT (default OFF). Verified: Linux + Windows(mingw)
build with chat ON, ctest 100%, hygiene clean; caches restored to the OFF default.

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

src/chat/chat_outgoing.cpp

@@ -0,0 +1,99 @@
+// DragonX Wallet - HushChat outgoing memo construction (implementation).
+
+#include "chat_outgoing.h"
+
+#include "chat_protocol.h"  // kHushChat* constants
+
+#include <nlohmann/json.hpp>
+
+namespace dragonx::chat {
+
+namespace {
+
+// Serialize the HushChat header. nlohmann emits object keys in sorted (alphabetical) order —
+// cid,e,h,p,t,v,z — which is exactly SilentDragonXLite's on-wire key order.
+std::string buildHeaderMemo(const std::string& replyZaddr,
+                            const std::string& conversationId,
+                            const char* type,
+                            const std::string& streamHeaderHex,
+                            const std::string& publicKeyHex)
+{
+    nlohmann::json header;
+    header["h"] = 1;                          // header number (>= 1)
+    header["v"] = kHushChatSupportedVersion;  // 0
+    header["z"] = replyZaddr;                 // where the peer should reply (my address)
+    header["cid"] = conversationId;
+    header["t"] = type;                       // "Memo" or "Cont"
+    header["e"] = streamHeaderHex;            // 48-hex secretstream header (Memo) / "" (Cont)
+    header["p"] = publicKeyHex;               // my 64-hex crypto_kx public key
+    return header.dump();
+}
+
+bool present(const std::string& value) { return !value.empty(); }
+
+} // namespace
+
+ChatComposeStatus buildOutgoingMessage(const ChatKeyPair& mine,
+                                       const std::string& myPublicKeyHex,
+                                       const std::string& myReplyZaddr,
+                                       const std::string& peerPublicKeyHex,
+                                       const std::string& peerZaddr,
+                                       const std::string& conversationId,
+                                       const std::string& plaintext,
+                                       OutgoingChatMemos& out)
+{
+    if (plaintext.empty()) return ChatComposeStatus::EmptyBody;
+    if (!present(myPublicKeyHex) || !present(myReplyZaddr) || !present(peerZaddr) ||
+        !present(conversationId)) {
+        return ChatComposeStatus::MissingField;
+    }
+    if (peerPublicKeyHex.size() != kHushChatPublicKeyHexLength) return ChatComposeStatus::BadPeerKey;
+
+    std::string streamHeaderHex;
+    std::string ciphertextHex;
+    if (encryptOutgoing(mine, peerPublicKeyHex, plaintext, streamHeaderHex, ciphertextHex)
+        != ChatCryptoStatus::Ok) {
+        return ChatComposeStatus::EncryptFailed;
+    }
+
+    OutgoingChatMemos memos;
+    memos.recipientZaddr = peerZaddr;
+    memos.headerMemo = buildHeaderMemo(myReplyZaddr, conversationId, "Memo", streamHeaderHex, myPublicKeyHex);
+    memos.payloadMemo = ciphertextHex;
+    if (memos.headerMemo.size() > kHushChatMemoByteLimit ||
+        memos.payloadMemo.size() > kHushChatMemoByteLimit) {
+        return ChatComposeStatus::TooLong;
+    }
+    out = std::move(memos);
+    return ChatComposeStatus::Ok;
+}
+
+ChatComposeStatus buildOutgoingContactRequest(const std::string& myPublicKeyHex,
+                                              const std::string& myReplyZaddr,
+                                              const std::string& peerZaddr,
+                                              const std::string& conversationId,
+                                              const std::string& requestText,
+                                              OutgoingChatMemos& out)
+{
+    if (requestText.empty()) return ChatComposeStatus::EmptyBody;
+    // The receive parser treats any memo starting with '{' as a header, so a request payload must
+    // not start with one (see isContactPayloadCandidate in chat_protocol.cpp).
+    if (requestText.front() == '{') return ChatComposeStatus::BadRequestText;
+    if (!present(myPublicKeyHex) || !present(myReplyZaddr) || !present(peerZaddr) ||
+        !present(conversationId)) {
+        return ChatComposeStatus::MissingField;
+    }
+
+    OutgoingChatMemos memos;
+    memos.recipientZaddr = peerZaddr;
+    memos.headerMemo = buildHeaderMemo(myReplyZaddr, conversationId, "Cont", "", myPublicKeyHex);
+    memos.payloadMemo = requestText;
+    if (memos.headerMemo.size() > kHushChatMemoByteLimit ||
+        memos.payloadMemo.size() > kHushChatMemoByteLimit) {
+        return ChatComposeStatus::TooLong;
+    }
+    out = std::move(memos);
+    return ChatComposeStatus::Ok;
+}
+
+} // namespace dragonx::chat