[Reply] Cap body at 1340 for desktop and tablet (flutter#425)

* Cap reply body with to 1340 on desktop/tablet

* Fix white lines around body of navigation rail

* Cap tablet width to 800

* Use Center widget instead of Row and Flexible

* Change divider width back to 1 and set cap at 1340 for both tablet and desktop

Co-authored-by: Pierre-Louis <6655696+guidezpl@users.noreply.github.com>

lib/studies/reply/adaptive_nav.dart

@@ -243,9 +243,14 @@ class _DesktopNavState extends State<_DesktopNav>
           ),
           const VerticalDivider(thickness: 1, width: 1),
           Expanded(
-            child: _SharedAxisTransitionSwitcher(
-              defaultChild: _MailNavigator(
-                child: MailboxBody(key: widget.inboxKey),
+            child: Center(
+              child: ConstrainedBox(
+                constraints: const BoxConstraints(maxWidth: 1340),
+                child: _SharedAxisTransitionSwitcher(
+                  defaultChild: _MailNavigator(
+                    child: MailboxBody(key: widget.inboxKey),
+                  ),
+                ),
               ),
             ),
           ),