action_sheet: Rename BottomSheetHeaderPlainText to BottomSheetInfoText

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 center of bottom sheet to give feedback to the user.

Additional changes:
  - Adds a TextAlign property for controlling the alignment.
  - Removes vertical padding so callers can decide about it. But keeps
    the horizontal padding of 16 as it is the default padding of our
    bottoms sheets.

Author: sm-sayedi

lib/widgets/action_sheet.dart

@@ -98,26 +98,31 @@ 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.
+/// Use it to present short, non-interactive explanatory messages to the user,
+/// such as an error message or other feedback.
+///
+/// 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 BottomSheetInfoText extends StatelessWidget {
+  const BottomSheetInfoText({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

@@ -773,8 +773,8 @@ 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: BottomSheetInfoText(text: zulipLocalizations.seeWhoReactedSheetNoReactions),
       );
     }