Add firmware slices 1–2: skeleton and Wi-Fi provisioning

Implement ESP-IDF walking skeleton with SoftAP, health API, and host
tests, then Slice 2 ISecureStore/NvsSecureStore, STA join, POST
/api/wifi, and the provisioning web UI.

Co-authored-by: Cursor <cursoragent@cursor.com>

Software/components/core/src/Secret.cpp

@@ -0,0 +1,64 @@
+/**
+ * @file    Secret.cpp
+ * @brief   Secret implementation.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/Secret.hpp"
+
+#include <cstring>
+
+namespace core {
+
+Secret::Secret(std::string value)
+    : bytes_(std::move(value))
+{
+}
+
+Secret::Secret(Secret&& other) noexcept
+    : bytes_(std::move(other.bytes_))
+{
+    other.zeroize();
+}
+
+Secret& Secret::operator=(Secret&& other) noexcept
+{
+    if (this != &other) {
+        zeroize();
+        bytes_ = std::move(other.bytes_);
+        other.zeroize();
+    }
+    return *this;
+}
+
+Secret::~Secret()
+{
+    zeroize();
+}
+
+std::size_t Secret::length() const noexcept
+{
+    return bytes_.size();
+}
+
+void Secret::zeroize() noexcept
+{
+    if (!bytes_.empty()) {
+        std::memset(bytes_.data(), 0, bytes_.size());
+        bytes_.clear();
+    }
+}
+
+} // namespace core