Fix WPF build error: remove PlaceholderText (WinUI 3 only)

PlaceholderText does not exist in WPF's TextBox. Removed from
RedirectUrlInput — the label above already describes what to paste.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ayangabryl

QuotaBarWindows/Views/AddAccountWindow.xaml

@@ -138,8 +138,7 @@
                     <TextBox x:Name="RedirectUrlInput" Background="#2C2C2E" Foreground="White"
                              BorderThickness="1" BorderBrush="#48484A"
                              Padding="10,8" FontSize="12" CaretBrush="White"
-                             TextWrapping="Wrap" AcceptsReturn="False" Height="60"
-                             PlaceholderText="https://console.anthropic.com/oauth/code/callback?code=..."/>
+                             TextWrapping="Wrap" AcceptsReturn="False" Height="60"/>
                     <TextBlock x:Name="PasteUrlError" Foreground="#FF453A" FontSize="11"
                                Margin="0,4,0,0" Visibility="Collapsed"/>
                     <Button x:Name="PasteUrlSubmit" Content="Connect"