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).

meson.build

@@ -26,6 +26,12 @@ if enable_tests
     subdir('tests')
 endif
 
+# Install the application icon into the hicolor theme: the waybar widget CSS
+# and any future .desktop file reference it.
+install_data('screencast_icon/screencast_256.png',
+    install_dir : get_option('datadir') / 'icons/hicolor/256x256/apps',
+    rename : 'screencast.png')
+
 # Summary for the user
 summary({
     'tests': enable_tests,