Switch theme from catppuccin to modus-vivendi

modus-vivendi is built into Emacs 29+ so it loads reliably without
a MELPA install — better for WSL where package errors were occurring.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

shared/.config/emacs/init.el

@@ -57,10 +57,10 @@
 
 ;;; Theme
 
-(use-package catppuccin-theme
+(use-package modus-themes
+  :ensure nil
   :config
-  (setq catppuccin-flavor 'mocha)
+  (load-theme 'modus-vivendi t))
-  (load-theme 'catppuccin t))
 
 ;;; Discoverability