feat(themeing): Major refactor themeing for whole computer

Adds catppuccin nix flake for themeing lots of programs easily. This
enables proper themeing for:
* zathura
* imv
* fcitx (declarative settings)
* lazygit
* btop
* gtk

It also pulls in the home-manager sessionVars which is necessary to
theme applications.

The session vars change also reduces the number of env-vars that need to
be set in the zshrc due to them being set declaratively elsewhere.

packages/default.nix

@@ -56,7 +56,6 @@ in
   config = {
     environment.systemPackages = (lib.attrVals [
       "black"
-      "btop"
       "clang-tools"
       "eza"
       "fd"
@@ -67,7 +66,6 @@ in
       "gnumake"
       "gnupg"
       "hugo"
-      "lazygit"
       "mypy"
       "neofetch"
       "nixd"