feat(chat): fee floor, real delivery status, pay-from-funded, funds pre-check

Chat sends move 0 value, so the network fee is structurally load-bearing (it's
the only thing that forces a real shielded input; 0-value + 0-fee builds a
degenerate, unrelayable tx). Three gaps addressed:

1. Fee floor — broadcastChatMemos now uses max(getDefaultFee(), kChatMinFeeDrgx),
   so a 0 / too-low global default-fee setting can't silently break chat.

2. Real delivery status — the echo was marked Sent on SUBMIT regardless of the
   on-chain outcome (the z_sendmany callback was empty), so failures were
   invisible and the Retry affordance never fired for async failures. Add a third
   ChatDelivery::Sending state (appended so persisted 0=Sent stays valid); record
   the echo in-memory as Sending, and resolve it to Sent/Failed from the
   z_sendmany completion callback — persisting only the final status (so a restart
   never shows a stuck spinner; a stray persisted Sending loads as Sent). A subtle
   "sending…" label shows while in flight.

3. Pay-from-funded + pre-check — z_sendmany spends from one z-address, and the
   identity reply address may be unfunded while funds sit elsewhere. chatPayFromZaddr
   picks a spendable z-address that can cover the fee (preferring the identity
   address); the memo still advertises the identity address as reply-to, so paying
   from a different note is transport-transparent. If nothing can cover the fee, a
   clear "need a small shielded balance" toast replaces the cryptic failure.

Full node only for the callback path; lite resolves optimistically on queue.
8-language strings + CJK subset (+1 glyph 賄).

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

src/app.h

@@ -717,9 +717,14 @@ private:
     void pollSweepStatus();              // Confirming step: poll sweep confirmations + legacy balance
     void beginAdoptSeedWallet();         // stop daemon -> swap wallet.dat -> restart with -rescan
     void provisionChatIdentityFromSecret(std::string secret);
-    std::string chatReplyZaddr();                                // a stable (persisted) wallet z-addr for chat
+    std::string chatReplyZaddr();                                // a stable (persisted) wallet z-addr — chat IDENTITY (reply-to)
+    // A spendable z-address that can actually PAY the fee (balance >= fee), preferring the identity
+    // reply address. The reply-to in the memo stays the identity address, so paying from a different
+    // funded note is transport-transparent. Empty if no z-address can cover the fee.
+    std::string chatPayFromZaddr(double fee) const;
     std::string generateChatLocalId(const char* prefix, int numBytes) const;  // unique echo id / cid
-    bool broadcastChatMemos(const chat::OutgoingChatMemos& memos);      // returns true if submitted
+    // Broadcast the memos and, when the async op resolves, flip the echo (echoLocalId) to Sent/Failed.
+    bool broadcastChatMemos(const chat::OutgoingChatMemos& memos, const std::string& echoLocalId);  // 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.