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>

res/lang/ko.json

@@ -162,9 +162,11 @@
     "chat_select_hint": "볼 대화를 선택하세요.",
     "chat_send": "전송",
     "chat_send_failed": "전송 안 됨",
+    "chat_sending": "전송 중…",
     "chat_time_now": "방금",
     "chat_toast_compose_failed": "메시지를 작성할 수 없습니다 (너무 긴가요?).",
     "chat_toast_lite_busy": "이미 전송이 진행 중이거나 열린 지갑이 없습니다.",
+    "chat_toast_need_funds": "채팅을 보내려면 수수료를 낼 소액의 보호 잔액이 필요합니다.",
     "chat_toast_no_zaddr": "채팅을 보낼 z-주소가 없습니다.",
     "chat_toast_not_connected": "연결되지 않음 — 채팅 메시지가 전송되지 않았습니다.",
     "chat_toast_request_compose_failed": "연락 요청을 작성할 수 없습니다 (잘못된 주소 / 텍스트?).",