Show app version in UI (welcome title + session sheet footer)

Reads the same stamped appVersion const the updater compares, so
the displayed version and the update logic can never disagree. Dev
runs show 0.0.0-dev.

apps/desktop/lib/src/screens/session_screen.dart

@@ -6,6 +6,7 @@ import 'package:sync_relay_core/core.dart';
 
 import '../session/health.dart';
 import '../state/session_state.dart';
+import '../update/updater.dart';
 import '../widgets/dialogs.dart';
 import '../widgets/member_avatar.dart';
 import '../widgets/window_buttons.dart';
@@ -681,6 +682,10 @@ class _SheetPanel extends StatelessWidget {
                     onPressed: onLeave,
                   ),
                 ),
+                const SizedBox(height: 8),
+                const Center(
+                  child: Text('v$appVersion', style: TextStyle(fontSize: 11, color: Colors.grey)),
+                ),
               ],
             );
           },

apps/desktop/lib/src/screens/welcome_screen.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 
 import '../state/session_state.dart';
+import '../update/updater.dart';
 import '../widgets/window_chrome_bar.dart';
 
 class WelcomeScreen extends StatelessWidget {
@@ -32,6 +33,7 @@ class _WelcomeBody extends StatelessWidget {
             mainAxisSize: MainAxisSize.min,
             children: [
               const Text('Watch Together', style: TextStyle(fontSize: 24)),
+              Text('v$appVersion', style: const TextStyle(fontSize: 12, color: Colors.grey)),
               const SizedBox(height: 8),
               const Text(
                 'Set your name + avatar on the next screen.',