vim:xdg: Fix updating spellfile through modeline

Setting `&spellfile` from the modeline is not allowed and will result in
an `E523: not allowed here` error. Fix the updating of the file by
additionally having an autocommand on BufWinEnter (i.e. after the
modeline was read).

TODO: Also get rid of the error message when rereading the modeline (:e)

.config/vim/xdg.vim

@@ -46,8 +46,15 @@ endif
 let &spellfile = $XDG_DATA_HOME . '/vim/spell/' . &spelllang . '.utf-8.add'
 augroup xdg_spellfile
 	au!
+	" TODO: This throws `E523: not allowed here` on `:e` as the modeline is
+	"       reread. Suppress or better check for sandbox/modeline before
+	"       executing
 	au OptionSet spelllang let &spellfile =
 				\ $XDG_DATA_HOME . '/vim/spell/' . v:option_new . '.utf-8.add'
+	" NOTE: Changing &spellfile is not allowed from a modeline, so we need to
+	"       update it manually after the modeline was read
+	au BufWinEnter * let &spellfile =
+				\ $XDG_DATA_HOME . '/vim/spell/' . &spelllang . '.utf-8.add'
 augroup end
 
 set runtimepath-=~/.vim       runtimepath^=$XDG_CONFIG_HOME/vim