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>

tests/test_phase4.cpp

@@ -5846,6 +5846,19 @@ void testHushChatDatabase()
         }
     }
 
+    // Delivery status persists (a failed outgoing echo round-trips as Failed).
+    {
+        ChatDatabase db(dbPath);
+        EXPECT_TRUE(db.unlockWithSecret("delivery-seed"));
+        ChatMessage m = makeMsg("tD", 1, "cD", "not-sent one", ChatMessageKind::Message, 555);
+        m.direction = ChatDirection::Outgoing;
+        m.delivery = ChatDelivery::Failed;
+        EXPECT_TRUE(db.append(m));
+        std::vector<ChatMessage> loaded = db.load();
+        EXPECT_EQ((int)loaded.size(), 1);
+        EXPECT_TRUE(loaded[0].delivery == ChatDelivery::Failed);
+    }
+
     scrub(dbPath); scrub(dbPath2);
 }