feat: non-blocking warmup — connect during daemon initialization

Instead of blocking the entire UI with "Activating best chain..." until
the daemon finishes warmup, treat warmup responses as a successful
connection. The wallet now:

- Sets connected=true + warming_up=true when daemon returns RPC -28
- Shows warmup status with block progress in the loading overlay
- Polls getinfo every few seconds to detect warmup completion
- Allows Console, Peers, Settings tabs during warmup
- Shows orange status indicator with warmup message in status bar
- Skips balance/tx/address refresh until warmup completes
- Triggers full data refresh once daemon is ready

Also: fix curl handle/header leak on reconnect, fill in empty
externalDetected error branch, bump version to v1.2.0 in build scripts.

src/rpc/rpc_client.h

@@ -53,6 +53,19 @@ public:
      */
     bool isConnected() const { return connected_; }
 
+    /**
+     * @brief True if the last connect() succeeded but daemon returned a warmup error.
+     * The curl handle is valid and auth succeeded — RPC calls will throw warmup errors
+     * until the daemon finishes initializing.
+     */
+    bool isWarmingUp() const { return warming_up_; }
+
+    /**
+     * @brief The warmup status message (e.g. "Activating best chain...").
+     * Empty when not in warmup.
+     */
+    const std::string& getWarmupStatus() const { return warmup_status_; }
+
     /**
      * @brief Get the error message from the last failed connect() attempt.
      */
@@ -182,6 +195,8 @@ private:
     std::string port_;
     std::string auth_;  // Base64 encoded "user:password"
     bool connected_ = false;
+    bool warming_up_ = false;
+    std::string warmup_status_;
     std::string last_connect_error_;
     mutable std::recursive_mutex curl_mutex_;  // serializes all curl handle access