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/util/i18n.cpp

@@ -217,6 +217,15 @@ void I18n::loadBuiltinEnglish()
         strings_["chat_you"] = "You";
         strings_["chat_contact_request"] = "contact request";
         strings_["chat_readonly_note"] = "Read-only — sending messages arrives in a later update.";
+        strings_["chat_new_button"] = "New conversation";
+        strings_["chat_select_hint"] = "Select a conversation to view it.";
+        strings_["chat_waiting_reply"] = "Waiting for this contact to reply — you can message them once they do.";
+        strings_["chat_send"] = "Send";
+        strings_["chat_new_title"] = "New conversation";
+        strings_["chat_new_zaddr"] = "Recipient z-address";
+        strings_["chat_new_message"] = "Message";
+        strings_["chat_new_send"] = "Send request";
+        strings_["chat_cancel"] = "Cancel";
         strings_["contacts_search_placeholder"] = "Search contacts...";
         strings_["contacts_search_no_match"] = "No matching contacts";
         strings_["address_book_confirm_delete"] = "Confirm delete?";