feat(chat): stamp sender compose-time in message header (clock-clamped)

Carry the sender's compose time as an optional "ts" (Unix seconds) in the
plaintext header JSON that rides outside the AEAD, and prefer it as the
displayed message time so both ends show the same send time regardless of when
the tx confirms. Parse "ts" leniently. On ingest, clamp: reject a "ts"
implausibly in the future vs the receive/block time (1h skew tolerated) so a
wrong/ahead peer clock can't pin messages to the bottom of a thread; a past
compose time is fine (the note buffer may broadcast a queued message later, and
a confirmed tx's block time is always >= compose time). Tests cover the
round-trip and the future-clock clamp.

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

src/chat/chat_protocol.h

@@ -23,6 +23,9 @@ struct HushChatHeader {
     HushChatHeaderType type = HushChatHeaderType::Message;
     std::string secretstream_header_hex;
     std::string public_key_hex;
+    // Optional sender-stamped compose time (header "ts", Unix seconds). 0 = absent (older sender) → the
+    // receiver falls back to the tx/receive time. Lets both sides show the SAME (send) time.
+    std::int64_t sent_at = 0;
 };
 
 struct HushChatHeaderParseResult {
@@ -80,6 +83,7 @@ struct HushChatTransactionMetadata {
     std::string sender_public_key_hex;   // header "p": peer crypto_kx public key (hex)
     std::string secretstream_header_hex; // header "e": secretstream header (hex; empty for ContactRequest)
     std::string payload_memo;            // ciphertext hex (Message) or plaintext request text (ContactRequest)
+    std::int64_t sent_at = 0;            // header "ts": sender compose time (Unix s); 0 = absent → use tx time
 };
 
 struct HushChatTransactionExtractionResult {