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/rpc/rpc_client.h

@@ -9,6 +9,7 @@
 #include <functional>
 #include <memory>
 #include <mutex>
+#include <stdexcept>
 #include <nlohmann/json.hpp>
 
 namespace dragonx {
@@ -18,6 +19,21 @@ using json = nlohmann::json;
 using Callback = std::function<void(const json&)>;
 using ErrorCallback = std::function<void(const std::string&)>;
 
+/**
+ * @brief A JSON-RPC error carrying the daemon's numeric error code.
+ *
+ * what() preserves the exact human-readable message (so existing string matching
+ * still works); `code` exposes the JSON-RPC error code — notably -28 (RPC_IN_WARMUP)
+ * for a daemon still starting up. Derives from std::runtime_error, so every existing
+ * `catch (const std::exception&)` continues to handle it unchanged.
+ */
+class RpcError : public std::runtime_error {
+public:
+    RpcError(int errorCode, const std::string& message)
+        : std::runtime_error(message), code(errorCode) {}
+    int code = 0;
+};
+
 /**
  * @brief JSON-RPC client for dragonxd
  *