Replace bland "W" initials with accent-colored person icon for signed-out state (#836)

When no Wino account is logged in, the titlebar button now shows a
filled accent-blue circle with a white person silhouette icon instead
of a PersonPicture with "W" initials. This makes the button visually
prominent and clearly communicates it's an account action. When signed
in, the PersonPicture with user initials is shown as before.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Wino.Mail.WinUI/ShellWindow.xaml

@@ -146,11 +146,25 @@
                                 </Grid>
                             </Flyout>
                         </Button.Flyout>
-                        <PersonPicture
+                        <Grid>
-                            x:Name="WinoAccountButtonPicture"
+                            <PersonPicture
-                            Width="30"
+                                x:Name="WinoAccountButtonPicture"
-                            Height="30"
+                                Width="30"
-                            Initials="W" />
+                                Height="30"
+                                Initials="W"
+                                Visibility="Collapsed" />
+                            <Border
+                                x:Name="WinoAccountSignedOutIcon"
+                                Width="30"
+                                Height="30"
+                                Background="{ThemeResource AccentFillColorDefaultBrush}"
+                                CornerRadius="15">
+                                <FontIcon
+                                    FontSize="16"
+                                    Foreground="White"
+                                    Glyph="&#xE77B;" />
+                            </Border>
+                        </Grid>
                     </Button>
                 </StackPanel>
             </TitleBar.RightHeader>

Wino.Mail.WinUI/ShellWindow.xaml.cs

@@ -372,6 +372,9 @@ public sealed partial class ShellWindow : WindowEx, IWinoShellWindow,
         WinoAccountSignedOutView.Visibility = isSignedIn ? Visibility.Collapsed : Visibility.Visible;
         WinoAccountSignedInView.Visibility = isSignedIn ? Visibility.Visible : Visibility.Collapsed;
 
+        WinoAccountButtonPicture.Visibility = isSignedIn ? Visibility.Visible : Visibility.Collapsed;
+        WinoAccountSignedOutIcon.Visibility = isSignedIn ? Visibility.Collapsed : Visibility.Visible;
+
         var initials = GetInitials(account?.Email);
 
         WinoAccountButtonPicture.Initials = initials;