text: Use proportionalLetterSpacing in the few existing places that want it

The letter spacings in these pieces of text hasn't been responding to system font-size adjustments. Now they do. There might not be any other instances of this problem in the app, because we widely apply 0 for [TextStyle.letterSpacing] (since 0065954), and when we do that, it shouldn't matter whether zero is considered as logical pixels or as a proportion the surrounding text size.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -667,7 +667,7 @@ class StreamMessageRecipientHeader extends StatelessWidget {
  final textStyle = TextStyle(
  color: contrastingColor,
  fontSize: 16,
- letterSpacing: 0.02 * 16,
+ letterSpacing: proportionalLetterSpacing(context, 0.02, baseFontSize: 16),
  height: (18 / 16),
  ).merge(weightVariableTextStyle(context, wght: 600));
 
@@ -775,9 +775,9 @@ class DmRecipientHeader extends StatelessWidget {
  child: Icon(size: 16, ZulipIcons.user)),
  Expanded(
  child: Text(title,
- style: const TextStyle(
+ style: TextStyle(
  fontSize: 16,
- letterSpacing: 0.02 * 16,
+ letterSpacing: proportionalLetterSpacing(context, 0.02, baseFontSize: 16),
  height: (18 / 16),
  ).merge(weightVariableTextStyle(context, wght: 600)),
  overflow: TextOverflow.ellipsis)),

lib/widgets/subscription_list.dart

@@ -148,7 +148,7 @@ class _SubscriptionListHeader extends StatelessWidget {
  style: TextStyle(
  color: const HSLColor.fromAHSL(1.0, 240, 0.1, 0.5).toColor(),
  fontSize: 14,
- letterSpacing: 0.04 * 14,
+ letterSpacing: proportionalLetterSpacing(context, 0.04, baseFontSize: 14),
  height: (16 / 14),
  ))),
  const SizedBox(width: 8),