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_database.cpp

@@ -250,6 +250,7 @@ std::string ChatDatabase::serialize(const ChatMessage& message) const
     json["b"] = message.body;
     json["ts"] = message.timestamp;
     json["pos"] = static_cast<std::uint64_t>(message.payload_position);
+    json["dl"] = static_cast<int>(message.delivery);
     return json.dump();
 }
 
@@ -266,6 +267,7 @@ bool ChatDatabase::deserialize(const std::string& json, ChatMessage& out) const
         out.body = parsed.value("b", std::string());
         out.timestamp = parsed.value("ts", static_cast<std::int64_t>(0));
         out.payload_position = static_cast<std::size_t>(parsed.value("pos", static_cast<std::uint64_t>(0)));
+        out.delivery = static_cast<ChatDelivery>(parsed.value("dl", 0));  // old rows → Sent
         return true;
     } catch (const std::exception&) {
         return false;