Fix modus-themes: let MELPA install it, revert to modus-vivendi

Removing :ensure nil so use-package fetches from MELPA — the built-in
version was missing or too old on this Emacs install.

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

shared/.config/emacs/init.el

@@ -58,9 +58,8 @@
 ;;; Theme
 
 (use-package modus-themes
-  :ensure nil
   :config
-  (load-theme 'modus-vivendi-tinted t))
+  (load-theme 'modus-vivendi t))
 
 ;;; Discoverability