feat(neovim): replace nixvim with programs.neovim (#336)

nixvim has been disabled for a long time (programs.nixvim.enable = false), so
drop the flake input (and prune flake.lock). Port the settings it carried to
Home Manager's programs.neovim: opts and per-plugin setup become an initLua
block plus a plugin list. The catppuccin module already provides the
colorscheme, vi stays unmapped, and the plain system neovim package is
untouched so sudo/nvim --clean keep using it.

flake.nix

@@ -53,11 +53,6 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
 
-    nixvim = {
-      url = "github:nix-community/nixvim/nixos-26.05";
-      inputs.nixpkgs.follows = "nixpkgs";
-    };
-
     # https://github.com/catppuccin/nix
     # main carries the rust-overlay-style deprecation fix for its vscode
     # package (nodejs -> nodejs-slim); v26.05 still emits the eval warning.