msglist [nfc]: Remove no-op Center widget in _MessageListPageState

The Center's child is a Column with the message list and possibly a
compose box.

Since the Column has the default MainAxisSize.max, it occupies all
available vertical space, so the Center is a no-op on the vertical
axis.

(If an element occupies all available space on an axis, it has only
one possible position on that axis, so the Center can't affect its
position.)

It turns out that the Column always takes all horizontal space, too;
the message-list and compose-box widgets both stretch to the full
screen width. This is appropriate because they want to handle the
horizontal device insets in separate ways: the former by keeping
visible UI outside the insets; the latter by filling the insets with
padding. So the Center is a no-op in the horizontal direction, too.

It's a relic from the very early prototype in late 2022 (see commit
b22ef67); remove it.

Also, with some new comments and dartdoc, make it clearer that the
Column takes the full screen width.

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1053,7 +1053,7 @@ class _ComposeBoxContainer extends StatelessWidget {
     };
 
     // TODO(design): Maybe put a max width on the compose box, like we do on
-    //   the message list itself
+    //   the message list itself; if so, remember to update ComposeBox's dartdoc.
     return Container(width: double.infinity,
       decoration: BoxDecoration(
         border: Border(top: BorderSide(color: designVariables.borderBar))),
@@ -1242,6 +1242,9 @@ class _ErrorBanner extends StatelessWidget {
   }
 }
 
+/// The compose box.
+///
+/// Takes the full screen width, covering the horizontal insets with its surface.
 class ComposeBox extends StatefulWidget {
   ComposeBox({super.key, required this.narrow})
     : assert(ComposeBox.hasComposeBox(narrow));

lib/widgets/message_list.dart

@@ -288,8 +288,10 @@ class _MessageListPageState extends State<MessageListPage> implements MessageLis
       //   we matched to the Figma in 21dbae120. See another frame, which uses that:
       //     https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=147%3A9088&mode=dev
       body: Builder(
-        builder: (BuildContext context) => Center(
-          child: Column(children: [
+        builder: (BuildContext context) => Column(
+          // Children are expected to take the full horizontal space
+          // and handle the horizontal device insets.
+          children: [
             MediaQuery.removePadding(
               // Scaffold knows about the app bar, and so has run this
               // BuildContext, which is under `body`, through
@@ -305,7 +307,7 @@ class _MessageListPageState extends State<MessageListPage> implements MessageLis
                 child: MessageList(narrow: narrow, onNarrowChanged: _narrowChanged))),
             if (ComposeBox.hasComposeBox(narrow))
               ComposeBox(key: _composeBoxKey, narrow: narrow)
-          ]))));
+          ])));
   }
 }
 
@@ -442,6 +444,11 @@ const _kShortMessageHeight = 80;
 // previous batch.
 const kFetchMessagesBufferPixels = (kMessageListFetchBatchSize / 2) * _kShortMessageHeight;
 
+/// The message list.
+///
+/// Takes the full screen width,
+/// keeping the message list and scroll-to-bottom button
+/// out of the horizontal insets with transparent [SafeArea] padding.
 class MessageList extends StatefulWidget {
   const MessageList({super.key, required this.narrow, required this.onNarrowChanged});
 
@@ -541,6 +548,8 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
       //   will have a `MediaQuery.removePadding` with `removeBottom: true`.
       bottom: false,
 
+      // Horizontally, on wide screens, this grows the SafeArea
+      // to position its padding over the device insets and centers content.
       child: Center(
         child: ConstrainedBox(
           constraints: const BoxConstraints(maxWidth: 760),