feat(chat): 0-conf fast-scan so incoming messages show before a block

The receive harvest gates each z-address behind scannedAtTip — it only re-scans
when a new block advances the tip — so incoming chat waited ~1 confirmation even
though the daemon already exposes mempool notes (FindMySaplingNotes runs on
mempool txs; z_listreceivedbyaddress(addr,0) returns them).

Add App::fastScanChatMemos(): every transaction-refresh cycle, re-scan JUST the
chat reply address (where peers send) at minconf=0, extract chat metadata, and
ingest — so messages surface at mempool speed (a few seconds) instead of waiting
for a block. Full-node only (lite has its own harvest). An in-flight guard avoids
stacking RPCs; the store dedups on txid+position, so the confirmed harvest never
double-inserts.

Hidden conversations are deliberately skipped by the fast path — they don't get
the mempool speed-up and still come back through the normal confirmed harvest
(which un-hides on a new message). New non-muted messages toast off-tab as usual.

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

src/app.cpp

@@ -1168,6 +1168,9 @@ void App::update()
                 } else if (walletDataPage && shouldRefreshRecentTransactions()) {
                     refreshRecentTransactionData();
                 }
+                // 0-conf chat: the normal harvest above only re-scans on a new block, so also do a light
+                // mempool scan of the chat address every cycle to surface messages before confirmation.
+                fastScanChatMemos();
             }
             if (network_refresh_.consumeDue(RefreshTimer::Addresses)) {
                 if (walletDataPage || addresses_dirty_ || hasTransactionSendProgress()) {