Add Slice 5 ADAU1701 runtime audio control (firmware 0.5.0).

Safeload mixer/EQ/master on the ADAU1701, persist AudioProfile in NVS,
expose /api/audio routes and web UI controls, with host tests and manual sync.

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

Software/components/secure_store/include/secure_store/NvsAudioProfileStore.hpp

@@ -0,0 +1,42 @@
+/**
+ * @file    NvsAudioProfileStore.hpp
+ * @brief   NVS-backed IAudioProfileStore for user audio settings.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+#pragma once
+
+#include "core/IAudioProfileStore.hpp"
+
+namespace secure_store {
+
+/**
+ * @brief    NvsAudioProfileStore — persists AudioProfile JSON in NVS.
+ *
+ * @dname    NvsAudioProfileStore
+ * @return   n/a (type)
+ * @pubstate Uses namespace digiradio, key audio_profile_json.
+ *
+ * @author   Michele Bigi
+ * @date     2026-07-06
+ */
+class NvsAudioProfileStore final : public core::IAudioProfileStore {
+public:
+    [[nodiscard]] bool hasProfile() const override;
+
+    [[nodiscard]] std::expected<void, core::StoreError> saveProfile(
+        const core::AudioProfile& profile) override;
+
+    [[nodiscard]] std::expected<core::AudioProfile, core::StoreError>
+    loadProfile() const override;
+
+    [[nodiscard]] std::expected<void, core::StoreError> clearProfile() override;
+};
+
+} // namespace secure_store