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.

users/home.nix

@@ -0,0 +1,36 @@
+{
+  ... }:
+let
+
+in {
+  catppuccin = {
+    enable = true;
+    flavor = "macchiato";
+    fcitx5.enable = false;
+    gtk.enable = true;
+    kvantum.enable = false;
+  };
+
+  qt = {
+    enable = true;
+    platformTheme.name = "qtct";
+  };
+
+  programs = {
+    btop = {
+      enable = true;
+      settings = {
+        theme_background = false;
+      };
+    };
+    imv.enable = true;
+    lazygit.enable = true;
+    wlogout.enable = true;
+    zathura = {
+      enable = true;
+      options = {
+        recolor-keephue = true;
+      };
+    };
+  };
+}