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.h

@@ -735,6 +735,7 @@ private:
     // (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
+    float chat_fast_scan_accum_ = 0.0f;       // seconds since the last fast-scan (dedicated ~2.5s poll)
     // 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.