feat: switch app icon with OS theme

[PavloGrubyi]

Summary

• add OS-theme app icon variants: white icon for dark OS theme, black icon for light OS theme
• update the GTK host window icon when the resolved OS color theme changes
• sync the user hicolor launcher icon named limux on startup and OS theme changes
• sync the per-user desktop entry Icon= to the explicit variant name so GNOME does not reuse a stale limux icon cache
• replace the existing tracked app icon PNG sizes with the new white/dark-theme fallback icon and include all generated PNG variants in the PR

Tests

• cargo fmt --check
• bash -n scripts/package.sh
• cargo +1.92 check -p limux-host-linux
• cargo +1.92 test -p limux-host-linux

Dogfood

• installed PR commit dcb3503 into ~/.local
• launched local GUI through gtk-launch dev.limux.linux on GNOME prefer-dark / Yaru-dark
• verified live control socket with limux identify --json
• verified installed desktop entry has Icon=limux-os-dark
• verified ~/.local/share/icons/hicolor/512x512/apps/limux.png and limux-os-dark.png are byte-identical white icons (opaque_rgb_mean=235.2)
• verified limux-os-light.png remains the black/light-theme icon (opaque_rgb_mean=51.2)