feat(chat): mark outgoing messages that fail to send

Outgoing echoes were recorded optimistically regardless of whether the broadcast
was actually submitted. Now broadcastChatMemos / broadcastChatMemosLite return
whether the send was submitted (false on immediate failures: not connected, no
spendable z-address, or a lite send already in progress), and the echo records a
ChatDelivery status (Sent / Failed). The Chat tab shows a "not sent" marker in
the error color on failed messages. The status persists in the DB (backward-
compatible: old rows deserialize as Sent).

(A later async failure — an opid that fails after submission — still surfaces via
the existing send-progress notification; only the submit gate is reflected here.)

Also removes the now-unused chat_readonly_note string (the composer replaced the
read-only footer). Test: delivery status round-trips through the DB. Verified:
Linux + Windows build with chat ON, ctest 100%, hygiene clean.

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

src/chat/chat_message.h

@@ -10,6 +10,9 @@ namespace dragonx::chat {
 
 enum class ChatDirection { Incoming, Outgoing };
 enum class ChatMessageKind { Message, ContactRequest };
+// Outgoing delivery: Sent = the broadcast was submitted; Failed = it wasn't (not connected, no
+// spendable address, a send already in progress). Always Sent for incoming.
+enum class ChatDelivery { Sent, Failed };
 
 struct ChatMessage {
     ChatDirection direction = ChatDirection::Incoming;
@@ -21,6 +24,7 @@ struct ChatMessage {
     std::string body;                 // decrypted plaintext (Message) or request text (ContactRequest)
     std::int64_t timestamp = 0;       // tx time in seconds; set by the ingesting caller
     std::size_t payload_position = 0; // together with txid, the dedup key
+    ChatDelivery delivery = ChatDelivery::Sent;  // outgoing only
 };
 
 } // namespace dragonx::chat