feat(app): wire the app icon into the GUI, waybar, and SDL window

screencast_icon/ holds the single source icon set; meson install ships
the 256 px PNG into the hicolor theme for the GTK panel (themed icon
name) and the waybar CSS, and a custom target embeds it as
icon_png_data.h so the SDL receiver window sets it via SDL_SetWindowIcon
with an optional SDL3_image dependency (built-in find_library, since no
pkg-config file ships with it).

scripts/icon_to_header.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+"""Generate a C++ header that embeds an image file as a byte array.
+
+Usage: icon_to_header.py INPUT_IMAGE OUTPUT_HEADER
+"""
+
+import sys
+
+
+def main() -> int:
+    if len(sys.argv) != 3:
+        print(f"usage: {sys.argv[0]} INPUT_IMAGE OUTPUT_HEADER", file=sys.stderr)
+        return 2
+
+    with open(sys.argv[1], "rb") as src:
+        data = src.read()
+
+    words = ", ".join(f"0x{byte:02x}" for byte in data)
+    header = f"""// GENERATED FILE - do not edit by hand.
+// Produced by scripts/icon_to_header.py from screencast_icon/screencast_256.png.
+
+#pragma once
+
+#include <array>
+#include <cstdint>
+
+namespace sc {{
+inline constexpr std::array<std::uint8_t, {len(data)}> app_icon_png = {{
+    {words}
+}};
+}}  // namespace sc
+"""
+    with open(sys.argv[2], "w") as dst:
+        dst.write(header)
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())