action_sheet [nfc]: Rename BottomSheetHeaderPlainText to BottomSheetPlainText

This way, it can be used for purposes other than being a header, such
as in the next commits, when read receipts fail to load, we use this
widget in the middle of bottom sheet to give feedback to the user.

This also adds a TextAlign property for controlling the alignment.

Author: sm-sayedi

lib/widgets/action_sheet.dart

@@ -98,26 +98,28 @@ void _showActionSheet(
     });
 }
 
-/// A header for a bottom sheet with a multiline UI string.
+/// A plain text widget for a bottom sheet with a multiline UI string.
 ///
-/// Assumes 8px padding below the top of the bottom sheet.
+/// Comes with built-in 16px horizontal padding.
 ///
 /// Figma:
 ///   https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=3481-26993&m=dev
-class BottomSheetHeaderPlainText extends StatelessWidget {
-  const BottomSheetHeaderPlainText({super.key, required this.text});
+class BottomSheetPlainText extends StatelessWidget {
+  const BottomSheetPlainText({super.key, required this.text, this.textAlign});
 
   final String text;
+  final TextAlign? textAlign;
 
   @override
   Widget build(BuildContext context) {
     final designVariables = DesignVariables.of(context);
 
     return Padding(
-      padding: EdgeInsets.fromLTRB(16, 8, 16, 4),
+      padding: EdgeInsets.symmetric(horizontal: 16),
       child: SizedBox(
         width: double.infinity,
         child: Text(
+          textAlign: textAlign,
           style: TextStyle(
             color: designVariables.labelTime,
             fontSize: 17,

lib/widgets/emoji_reaction.dart

@@ -792,8 +792,8 @@ class ViewReactionsHeader extends StatelessWidget {
 
     if (reactions == null || reactions.aggregated.isEmpty) {
       return Padding(
-        padding: const EdgeInsets.only(top: 8),
-        child: BottomSheetHeaderPlainText(text: zulipLocalizations.seeWhoReactedSheetNoReactions),
+        padding: const EdgeInsets.only(top: 16, bottom: 4),
+        child: BottomSheetPlainText(text: zulipLocalizations.seeWhoReactedSheetNoReactions),
       );
     }