Custom waterfall colors (#2617)

* Custom waterfall gradient
* Installing a custom waterfall gradient via fileman
* default file for user friendly swap
Co-authored-by: gullradriel <gullradriel@no-mail.com>

firmware/application/gradient.hpp

@@ -0,0 +1,51 @@
+/*
+ * Copyright (C) 2025 Belousov Oleg
+ *
+ * This file is part of PortaPack.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#ifndef __GRADIENT_H__
+#define __GRADIENT_H__
+
+#include "ui.hpp"
+#include "file.hpp"
+
+#include <array>
+#include <cstdint>
+
+extern const std::filesystem::path default_gradient_file;
+
+class Gradient {
+   public:
+    std::array<ui::Color, 256> lut{};
+
+    Gradient();
+
+    void set_default();
+    bool load_file(const std::filesystem::path& file_path);
+
+   private:
+    int16_t prev_index = 0;
+    int16_t prev_r = 0;
+    int16_t prev_g = 0;
+    int16_t prev_b = 0;
+
+    void step(int16_t index, int16_t r, int16_t g, int16_t b);
+};
+
+#endif /* __GRADIENT_H__ */