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/app.h

@@ -602,8 +602,8 @@ private:
     void provisionChatIdentityFromSecret(std::string secret);
     std::string chatReplyZaddr();                                // a stable (persisted) wallet z-addr for chat
     std::string generateChatLocalId(const char* prefix, int numBytes) const;  // unique echo id / cid
-    void broadcastChatMemos(const chat::OutgoingChatMemos& memos);      // full-node z_sendmany / lite
-    void broadcastChatMemosLite(const chat::OutgoingChatMemos& memos);  // lite two-recipient send
+    bool broadcastChatMemos(const chat::OutgoingChatMemos& memos);      // returns true if submitted
+    bool broadcastChatMemosLite(const chat::OutgoingChatMemos& memos);  // lite two-recipient send
     void ingestLiteChatMemos(const wallet::LiteWalletAppRefreshModel& model);  // lite chat receive harvest
     // Lite first-run welcome prompt: dismissed for the session once the user picks an action.
     bool lite_firstrun_dismissed_ = false;