dialog: Start-align dialog content on iOS, instead of center-align

We're about to show a stack trace in an error dialog, and it'll be
easier to read when start-aligned than center-aligned.

Applying it here with DefaultTextStyle, rather than on each Text
widget, to avoid repetition, particularly because one caller (of
_adaptiveContent) passes multiple Text widgets and we'd have to
handle each of those; that's in UpgradeWelcomeDialog.

Author: chrisbobbe

lib/widgets/dialog.dart

@@ -63,7 +63,14 @@ Widget? _adaptiveContent(Widget? content) {
     case TargetPlatform.macOS:
       // A [SingleChildScrollView] (wrapping both title and content) is already
       // created by [CupertinoAlertDialog].
-      return content;
+      return DefaultTextStyle.merge(
+        // The "alert description" is start-aligned in one example in Apple's
+        // HIG document:
+        //   https://developer.apple.com/design/human-interface-guidelines/alerts#Anatomy
+        // (Confusingly, in 2025-10, it's center-aligned in the graphic at the
+        // *top* of that page; shrug.)
+        textAlign: TextAlign.start,
+        child: content);
   }
 }