catppuccin/nix + stylix: disabling a stylix target kills its transparency too (btop, cava)

When catppuccin/nix and stylix are both used, the usual fix for a clash (e.g. programs.btop.settings.color_theme) is stylix.targets.btop.enable = false. Watch out: that also drops stylix's opacity handling. Stylix's btop target sets theme_background = false when stylix.opacity.terminal != 1.0, so btop stops being transparent.

Fix: turn off only the colors part. Recent stylix mkTarget targets have sub-toggles, so use stylix.targets.btop.colors.enable = false (same for ghostty: ghostty.colors.enable = false keeps fonts and background-opacity).

Cava: catppuccin's default cava theme has a solid background. Set catppuccin.cava.transparent = true to get the <flavor>-transparent.cava file. That option exists only in the home-manager module; putting it in a block shared with the NixOS module fails with "The option `catppuccin.cava' does not exist". Also disable stylix's cava target, otherwise its gradient_color_* values mix with catppuccin's theme.