msglist: Set message-sender-name color to match web

I overlooked this in zulip#686 because this text's color wasn't set at
this site explicitly, and my process for that PR was to search
case-insensitively for the term [color]. Instead of being set
locally, this text's color was being set by the DefaultTextStyle
that wraps the whole message list for the sake of coloring message
content.

That color doesn't match web, it turns out. (Or the old Figma, which
uses true black.) Web colors this with
var(--color-text-sender-name), but that variable is undefined for
light theme, so it falls back to --color-text-default which is this
color.

At this commit, if I change the message list's DefaultTextStyle to
set a conspicuous color, like orange, I only see a response from
message content, and not from sender names or anything else. That's
good, but I wonder if there's a less error-prone solution for
coloring just the message content, without regressing on
performance. I'll explore that later in this series.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -915,10 +915,11 @@ class MessageWithPossibleSender extends StatelessWidget {
                   const SizedBox(width: 8),
                   Flexible(
                     child: Text(message.senderFullName, // TODO get from user data
-                      style: const TextStyle(
+                      style: TextStyle(
                         fontFamily: 'Source Sans 3',
                         fontSize: 18,
                         height: (22 / 18),
+                        color: const HSLColor.fromAHSL(1, 0, 0, 0.2).toColor(),
                       ).merge(weightVariableTextStyle(context, wght: 600)),
                       overflow: TextOverflow.ellipsis)),
                   if (sender?.isBot ?? false) ...[