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/util/i18n.cpp

@@ -216,7 +216,7 @@ void I18n::loadBuiltinEnglish()
         strings_["chat_empty_hint"] = "No conversations yet. Messages you receive will appear here.";
         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_send_failed"] = "not sent";
         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.";