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),