Stylix helix target: keep custom tweaks via inherits stylix

Stylix's helix target sets programs.helix.settings.theme = "stylix" and writes themes.stylix as a file, so you cannot merge attrs into it and a second settings.theme definition conflicts. To keep per-mode tweaks, define themes.<name> = { inherits = "stylix"; "ui.statusline.normal" = { fg = "base00"; bg = "base0A"; }; } and set settings.theme = lib.mkForce "<name>". The base16-helix template exposes palette names base00..base0F, and with stylix.opacity.terminal != 1.0 it auto-strips the helix background (transparent). For bat, enabling the stylix bat target sets programs.bat.config.theme = "base16-stylix", so drop any --theme flag in fzf preview commands.