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/account/logout.html

@@ -26,7 +26,7 @@
         {{ redirect_field }}
 
         <div class="flex flex-wrap items-center justify-end gap-2">
-            <a class="btn gap-2" href="/">{% lucide "arrow-left" size=16 %} {% trans "Cancel" %}</a>
+            <a class="btn btn-outline gap-2" href="/">{% lucide "arrow-left" size=16 %} {% trans "Cancel" %}</a>
             <button class="btn btn-primary gap-2" type="submit">{% lucide "log-out" size=16 %} {% trans "Sign Out" %}</button>
         </div>
     </form>