Lightbox: Added user's avatar in the lightbox app bar

This change updates the lightbox to display the message author's avatar
alongside their name and the date in the app bar. The avatar is positioned
in the "start" direction (left for LTR layouts, right for RTL layouts) to
align with the behavior in the React Native app.

Fixes #41

lib/widgets/lightbox.dart

@@ -172,21 +172,27 @@ class _LightboxPageLayoutState extends State<_LightboxPageLayout> {
         shape: const Border(), // Remove bottom border from [AppBarTheme]
         elevation: appBarElevation,
 
-        // TODO(#41): Show message author's avatar
-        title: RichText(
-          text: TextSpan(children: [
-            TextSpan(
-              text: '${widget.message.senderFullName}\n',
-
-              // Restate default
-              style: themeData.textTheme.titleLarge!.copyWith(color: appBarForegroundColor)),
-            TextSpan(
-              text: timestampText,
-
-              // Make smaller, like a subtitle
-              style: themeData.textTheme.titleSmall!.copyWith(color: appBarForegroundColor)),
-          ])),
-        bottom: widget.buildAppBarBottom(context));
+        title: Row(
+          children: [
+            Avatar(size: 40, borderRadius: 32 / 8, userId: widget.message.senderId),
+            const SizedBox(width: 8),
+            Expanded(
+              child: RichText(
+                text: TextSpan(children: [
+                  TextSpan(
+                      text: '${widget.message.senderFullName}\n',
+                      // Restate default
+                      style: themeData.textTheme.titleLarge!.copyWith(color: appBarForegroundColor)),
+                  TextSpan(
+                      text: timestampText,
+                      // Make smaller, like a subtitle
+                      style: themeData.textTheme.titleSmall!.copyWith(color: appBarForegroundColor)),
+                ])),
+            ),
+          ],
+        ),
+        bottom: widget.buildAppBarBottom(context)
+      );
     }
 
     Widget? bottomAppBar;