fix(rescan): stop the instant false "rescan complete"; show live status

Clicking Settings → Rescan restarted the daemon with -rescan correctly, but the
progress poll fired "Blockchain rescan complete" the instant it was clicked,
then showed nothing for the entire (multi-hour) rescan — so it looked broken.

Cause: the very first getrescaninfo poll runs before the daemon has restarted
and hits the still-running pre-restart daemon, which answers rescanning=false.
The completion branch took that as "done", cleared the rescanning flag, and the
real rescan then ran invisibly. (Confirmed from a Windows debug-log capture: an
instant OK{"rescanning":false}, then ~6400 warmup errors over ~5h, all swallowed.)

Fixes:
- Gate completion on a new rescan_confirmed_active_ flag that's only set once we
  actually observe the rescan running, so a pre-restart rescanning=false can't be
  misread as completion.
- While the daemon is in -rescan RPC warmup it rejects every call with the live
  phase as the message ("Loading block index..." -> "Rescanning..."). Treat that
  as proof-of-progress: surface it as rescan_status and mark confirmed-active,
  instead of silently swallowing it. The status bar keeps its animated
  "Rescanning..." for the whole run, then reports complete when warmup ends.
- Read rescan_progress whether the daemon returns it as a string or a number
  (the get<std::string>() would have thrown on a numeric field).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.h

@@ -595,6 +595,10 @@ private:
     bool transactions_dirty_ = false;       // true → force tx refresh regardless of block height
     bool encryption_state_prefetched_ = false; // suppress duplicate getwalletinfo on connect
     bool rescan_status_poll_in_progress_ = false;
+    // True once we've actually observed the rescan running (daemon restarted into -rescan warmup).
+    // Gates the "rescan complete" detection so a getrescaninfo poll that hits the still-running
+    // pre-restart daemon (which reports rescanning=false) can't fire a false "complete" instantly.
+    bool rescan_confirmed_active_ = false;
     bool opid_poll_in_progress_ = false;
     // Consecutive Core-refresh cycles where BOTH core RPCs failed → likely a dead
     // connection. After kCoreFailuresBeforeDisconnect, tear down and reconnect.