fix: apply the standalone appearance preference across the app

[alexandropaul]

The appearance preference added in f0cc7c0 referenced set_app_appearance, which was never defined, so the build failed. This wires it up and makes the setting reach every layer.

• set_app_appearance now toggles both gtk_application_prefer_dark_theme and the StyleManager color scheme, and re-applies terminal colors on change.
• The "auto" terminal palette follows this preference instead of the desktop-wide theme mode, so a forced light/dark reaches the terminal outside the shell.
• Leafs' stylesheet is loaded above the framework theme so the hover controls stay dark and the Theme selector list follows the window in light mode.
• The installed binary embeds an rpath to the prefix libdir so it launches from the app menu without LD_LIBRARY_PATH.