feat(chat): pre-split note buffer for rapid sends + status + console logging

Each chat message is a shielded tx that spends a verified note, so a burst of
messages hit "insufficient verified funds" once the single note is spent. Add a
per-frame note-buffer coordinator (both variants) that keeps ~10 verified
spendable notes: it serializes sends through the one broadcast channel, counts
verified notes by block depth (lite) or a rate-limited z_listunspent scan
(full node), self-splits in the background to refill toward the target, and
queues overflow to drain as change matures — with honest Sent/Failed status
instead of the prior optimistic "Sent". Guardrails: single split in flight with
a watchdog, cooldown, and session-generation guards so a wallet switch can't
drain another wallet's queue. Surface a "Chat buffer: N/10 ready" indicator in
the status bar while the Chat tab is active, and route chat diagnostics through
the console on both variants (the full-node console now drains the shared ring).

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

src/ui/windows/console_command_executor.h

@@ -113,6 +113,9 @@ private:
     bool   last_rpc_connected_      = false;
     std::deque<std::pair<std::string, bool>> results_;  // {text, isError}
     std::mutex results_mutex_;
+    // Chat diagnostics ring cursor: chat code logs via wallet::liteLog on BOTH variants, so the full-node
+    // console surfaces those lines (as App messages) too — mirrors LiteConsoleExecutor's diag drain.
+    std::uint64_t diag_gen_ = static_cast<std::uint64_t>(-1);
 };
 
 // ── Lite: diagnostics ring + backend console command ─────────────────────────