ui: Follow Figma in adding app-bar bottom border

The default app-bar color is now extremely similar to the scaffold
background color -- Color(0xfff5f5f5) and Color(0xfff6f6f6),
respectively -- and both follow the Figma. Because of their
similarity, it seems helpful to delineate the app bar a bit better,
and we can do so by adding this bottom border, which the Figma has:
  https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?type=design&node-id=143-10772&mode=design&t=Nvsl6UdRBQvH3zXV-0

In the message list, the Figma isn't consistent on whether to omit
the border when the app bar matches the color of all the recipient
headers, like in a stream narrow. I think it's better to omit it in
those cases, so I've done that here.

lib/widgets/app.dart

@@ -91,6 +91,8 @@ class ZulipApp extends StatelessWidget {
         // ColorScheme.surface otherwise, and those are different colors.
         scrolledUnderElevation: 0,
         backgroundColor: Color(0xfff5f5f5),
+
+        shape: Border(bottom: BorderSide(color: Color(0xffcccccc))),
       ),
       // This applies Material 3's color system to produce a palette of
       // appropriately matching and contrasting colors for use in a UI.

lib/widgets/message_list.dart

@@ -58,20 +58,33 @@ class _MessageListPageState extends State<MessageListPage> {
     final store = PerAccountStoreWidget.of(context);
 
     final Color? backgroundColor;
+    bool removeAppBarBottomBorder = false;
     switch(widget.narrow) {
       case AllMessagesNarrow():
         backgroundColor = null; // i.e., inherit
+
       case StreamNarrow(:final streamId):
       case TopicNarrow(:final streamId):
         backgroundColor = store.subscriptions[streamId]?.colorSwatch().barBackground
           ?? _kUnsubscribedStreamRecipientHeaderColor;
+        // All recipient headers will match this color; remove distracting line
+        // (but are recipient headers even needed for topic narrows?)
+        removeAppBarBottomBorder = true;
+
       case DmNarrow():
         backgroundColor = _kDmRecipientHeaderColor;
+        // All recipient headers will match this color; remove distracting line
+        // (but are recipient headers even needed?)
+        removeAppBarBottomBorder = true;
     }
 
     return Scaffold(
       appBar: AppBar(title: MessageListAppBarTitle(narrow: widget.narrow),
-        backgroundColor: backgroundColor),
+        backgroundColor: backgroundColor,
+        shape: removeAppBarBottomBorder
+          ? const Border()
+          : null, // i.e., inherit
+      ),
       // TODO question for Vlad: for a stream view, should we set
       //   [backgroundColor] based on stream color, as in this frame:
       //     https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=132%3A9684&mode=dev