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.h

@@ -730,6 +730,11 @@ private:
     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
+    // Full-node 0-conf fast path: re-scan just the chat reply address at minconf=0 every refresh cycle
+    // so incoming messages surface at mempool speed (before a block). Hidden conversations are skipped
+    // (they still un-hide via the normal confirmed harvest). Self-gated; no-op without a chat identity.
+    void fastScanChatMemos();
+    bool chat_fast_scan_in_flight_ = false;   // guard against overlapping fast-scan RPCs
     // Lite first-run welcome prompt: dismissed for the session once the user picks an action.
     bool lite_firstrun_dismissed_ = false;
     // Lite send-time unlock: set to show the unlock modal when a spend is attempted while locked.