feat(chat): dedicated ~2.5s poll for the 0-conf fast-scan

The fast-scan was hung on the page's Transactions timer, which is 10–15s on most
pages (15s on Chat), so incoming messages still took up to ~15s to appear. Give it
its own ~2.5s accumulator (delta-time based, independent of the page cadence) so
messages land in a few seconds — network propagation then dominates, which is as
fast as 0-conf gets. Still gated behind the warmup/rescan block and self-gated in
fastScanChatMemos; the in-flight guard prevents overlapping RPCs.

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

src/app.cpp

@@ -1168,8 +1168,14 @@ 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.
+            }
+            // 0-conf chat: a DEDICATED ~2.5s mempool scan of the chat address, independent of the page's
+            // (slower, 10–15s) Transactions cadence — so incoming messages surface in a few seconds. The
+            // normal harvest only re-scans on a new block. Self-gated (no-op without a chat identity /
+            // connection / on lite); the in-flight guard keeps overlapping RPCs from stacking.
+            chat_fast_scan_accum_ += ImGui::GetIO().DeltaTime;
+            if (chat_fast_scan_accum_ >= 2.5f) {
+                chat_fast_scan_accum_ = 0.0f;
                 fastScanChatMemos();
             }
             if (network_refresh_.consumeDue(RefreshTimer::Addresses)) {