msglist: Pad left and right insets

On some small devices in landscape mode, the `maxWidth: 760`
constraint doesn't (and isn't meant to) prevent the content from
extending into the left and right insets. So, use a SafeArea widget
for that.

In contrast to the SafeAreaView component from
react-native-safe-area-context in the RN app, this SafeArea widget:
  - knows when an ancestor has declared the given inset(s) consumed
    (by reading the ambient MediaQueryData.padding), and
  - declares the given inset(s) consumed (by changing its
    descendants' ambient MediaQueryData.padding).

It's left unaware of non-ancestors that consume insets. It turns out
that some insets are consumed by non-ancestors in this case; study
how that's done and add a few lines to adapt.

See the doc:
  https://api.flutter.dev/flutter/widgets/SafeArea-class.html

The `minimum: 8` should preserve the existing 8px padding in all the
places we want it, including:
  - phones with no notches/etc.
  - notched phones in portrait mode where the norm is to have no
    notches/etc. on the left or right.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -66,8 +66,22 @@ class _MessageListState extends State<MessageList> {
         child: ColoredBox(
             color: Colors.white,
             child: Center(
-              child: Padding(
-                  padding: const EdgeInsets.symmetric(horizontal: 8),
+              // Pad the left and right insets, for small devices in landscape.
+              child: SafeArea(
+                  // A non-ancestor (the compose box) pads the bottom inset;
+                  // prevent extra padding here.
+                  bottom: false,
+
+                  // A non-ancestor (the app bar) pads the top inset. But no
+                  // need to prevent extra padding with `top: false`, because
+                  // Scaffold, which knows about the app bar, sets `body`'s
+                  // ambient `MediaQueryData.padding` to have `top: 0`:
+                  //   https://github.com/flutter/flutter/blob/3.7.0-1.2.pre/packages/flutter/lib/src/material/scaffold.dart#L2778
+
+                  // Keep some padding when there are no horizontal insets,
+                  // which is usual in portrait mode.
+                  minimum: const EdgeInsets.symmetric(horizontal: 8),
+
                   child: ConstrainedBox(
                       constraints: const BoxConstraints(maxWidth: 760),
                       child: _buildListView(context))))));