Make every cancel/back button an outline button

The filled base button reads as heavy as the primary next to it, so cancel is now
btn-outline everywhere it appears: sign out, the 2FA challenge, and the four
control-panel forms (which also gain the back-arrow icon the auth pages already
had).

For the record, the buttons were never different heights -- measured in a real
browser, every .btn on every page is 40px, anchor and button alike. It was the
fill, not the box.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/mfa/authenticate.html

@@ -33,7 +33,7 @@
         </div>
 
         <div class="flex flex-wrap items-center justify-end gap-2">
-            <button class="btn gap-2" type="submit" form="logout-from-stage">{% lucide "x" size=16 %} {% trans "Cancel" %}</button>
+            <button class="btn btn-outline gap-2" type="submit" form="logout-from-stage">{% lucide "x" size=16 %} {% trans "Cancel" %}</button>
             <button class="btn btn-primary gap-2" type="submit">{% lucide "log-in" size=16 %} {% trans "Sign In" %}</button>
         </div>
     </form>