[Emacs] Fix evaluation issue of `mu4e-headers-visible-columns' eval

This commit fixes a bug introduced in 8c6e622a7c and 69dc8c7d7c where
the mu4e-headers view’s width was set to a dynamic, frame-dependant
value that could not be computed with Emacs launched as a headless
server.

This commit makes this value evaluate when the `mu4e-headers' mode is
activated, and hence it should get the correct width of the current
frame.

org/config/spacemacs.org

@@ -3224,12 +3224,16 @@
    On modern-day  computers, with  wide screens almost  everywhere, there  is no
    reason for the email buffer to open  below the email directory and not on its
    right,  which  is  why I  set  the  split  view  to be  vertical  instead  of
-   horizontal. And to make it more readable, the header window will only occupy
-   40% of Emacs’ frame, the rest will be given to emails.
+   horizontal. And to make it more  readable, the header window will only occupy
+   40% of Emacs’  frame, the rest will be  given to emails. As you  can see, the
+   width of the mu4e headers is evaluated each time we enter it, so it can react
+   to the frame being potentially not the same width than earlier.
    #+BEGIN_SRC emacs-lisp
-     (setq mu4e-split-view 'vertical
-           mu4e-headers-visible-columns (round (* (frame-total-cols)
-                                                  0.4)))
+     (setq mu4e-split-view 'vertical)
+     (add-hook 'mu4e-headers-mode-hook
+               (lambda ()
+                 (setq mu4e-headers-visible-columns (round (* (frame-total-cols)
+                                                              0.4)))))
    #+END_SRC
 
    This  is the  setup I  have for  my  SMTP mail  server: I  point Emacs’  SMTP