fix(rpc): detect mid-session disconnects and stop blocking the UI thread

The connection state machine never tore down on a lost connection: refresh-loop
RPC errors were swallowed, rpc_->isConnected() stayed true after a daemon
crash/restart/socket drop, and the UI showed stale balances with no reconnect.
Several operations also ran synchronous curl straight from ImGui handlers.

- Add handleLostConnection(): after N consecutive cycles where BOTH core RPCs
  fail (warmup excluded, so no reconnect loop), disconnect so update()'s
  reconnect branch re-enters tryConnect().
- Move banPeer/unbanPeer/clearBans and key export/import onto the worker thread
  (import requests a rescan that could freeze the UI for the curl timeout).
- Run the block-info dialog's two chained RPCs on the worker thread (+ guard the
  getblockhash result type).
- Detect daemon warmup via the JSON-RPC -28 code (new RpcError carrying the code;
  message text preserved so 401/warmup string-matching is unaffected), and widen
  CONNECTTIMEOUT to 10s for remote/TLS hosts (2s localhost).

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

src/app.h

@@ -574,6 +574,9 @@ private:
     bool encryption_state_prefetched_ = false; // suppress duplicate getwalletinfo on connect
     bool rescan_status_poll_in_progress_ = 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.
+    int consecutive_core_failures_ = 0;
 
     // Pending z_sendmany operation tracking
     bool send_progress_active_ = false;
@@ -692,6 +695,10 @@ private:
     void tryConnect();
     void onConnected();
     void onDisconnected(const std::string& reason);
+    // Tear down a connection that died mid-session (daemon crash / restart / dropped
+    // socket) so update()'s reconnect branch re-enters tryConnect(). Unlike onDisconnected
+    // alone, this also rpc_->disconnect()s so rpc_->isConnected() actually flips to false.
+    void handleLostConnection(const std::string& reason);
     void applyDefaultBanlist();
 
     // Private methods - data refresh