Make desktop error banners selectable and copyable

The error banner used WinUI's nonselectable message presentation, preventing
users from copying diagnostics. Keep the native banner while rendering its
message in a selectable text control.

Replace the message body with wrapped, scrollable text and add a Copy action.
Copy uses the full message and reports success separately. Clipboard failures
preserve the original error and offer text selection as a fallback; a new error
clears prior copy feedback. The text exposes its contents to accessibility tools
and marks errors as assertive accessibility updates.

Users can copy either selected text or the complete diagnostic without
transcribing it, including long messages and Unicode paths.

The ARM64 single-file desktop build passed. UI automation selected a real
validation error containing a Unicode path and verified that Copy produced
exactly the displayed text. Holding the Windows clipboard open confirmed that
copy failure retained the error and showed the fallback. The banner was
visually inspected. No Rust behavior changed.

Design-Signed-Off-By: Codex (GPT-6); design-review=cfcf

Author: lukealonso

flare-winui/Shell.xaml.h

@@ -14,7 +14,10 @@ inline constexpr wchar_t shell_xaml[] = LR"xaml(
   <NavigationView.PaneFooter><StackPanel Margin="18,0,12,18" Spacing="5"><TextBlock Text="THIS PC" FontSize="10" CharacterSpacing="100" Style="{StaticResource Secondary}"/><TextBlock x:Name="DeviceName" Text="Device details pending" FontSize="12" TextWrapping="Wrap"/><TextBlock x:Name="DeviceState" Text="Checked before loading" Style="{StaticResource Caption}"/></StackPanel></NavigationView.PaneFooter>
   <Border Background="{ThemeResource WorkspaceBrush}" BorderBrush="{ThemeResource StrokeBrush}" BorderThickness="1,1,0,0" CornerRadius="4,0,0,0"><Grid x:Name="Workspace" Padding="24" RowSpacing="0"><Grid.RowDefinitions><RowDefinition Height="Auto"/><RowDefinition Height="Auto"/><RowDefinition Height="*"/></Grid.RowDefinitions>
    <Grid x:Name="PageHeader" Margin="0,0,0,20"><Grid.ColumnDefinitions><ColumnDefinition Width="*"/><ColumnDefinition Width="Auto"/></Grid.ColumnDefinitions><TextBlock x:Name="PageTitle" Text="Overview" FontSize="28" FontWeight="SemiBold"/><StackPanel Grid.Column="1" Orientation="Horizontal" Spacing="8" VerticalAlignment="Top"><Button x:Name="RefreshButton" Content="Refresh" AutomationProperties.Name="Refresh model library"/><Button x:Name="AddModelButton" Content="＋ Add model"/></StackPanel></Grid>
-   <InfoBar x:Name="Notice" Grid.Row="1" IsOpen="False" IsClosable="True" Severity="Error"/><Grid Grid.Row="2">
+   <InfoBar x:Name="Notice" Grid.Row="1" IsOpen="False" IsClosable="True" Severity="Error">
+    <InfoBar.ActionButton><Button x:Name="CopyErrorButton" Content="Copy" AutomationProperties.Name="Copy error message" ToolTipService.ToolTip="Copy the full error message"/></InfoBar.ActionButton>
+    <StackPanel Spacing="8" Margin="0,0,0,12"><ScrollViewer MaxHeight="240" VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Disabled" HorizontalScrollMode="Disabled"><TextBlock x:Name="NoticeMessage" IsTextSelectionEnabled="True" TextWrapping="Wrap" AutomationProperties.LiveSetting="Assertive"/></ScrollViewer><TextBlock x:Name="ErrorCopyStatus" FontSize="12" TextWrapping="Wrap" Visibility="Collapsed" AutomationProperties.LiveSetting="Polite"/></StackPanel>
+   </InfoBar><Grid Grid.Row="2">
 )xaml"
 LR"xaml(
 <ScrollViewer x:Name="OverviewPage" VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Disabled" HorizontalScrollMode="Disabled" HorizontalContentAlignment="Stretch"><StackPanel Spacing="14">

flare-winui/main.cpp

@@ -159,7 +159,9 @@ struct DesktopApp : ApplicationT<DesktopApp, Markup::IXamlMetadataProvider> {
     void show_error(std::wstring const& message) {
         auto notice = control<InfoBar>(L"Notice");
         notice.Title(L"Flare could not complete the operation");
-        notice.Message(message);
+        text(L"NoticeMessage",message);
+        text(L"ErrorCopyStatus",L"");
+        control<TextBlock>(L"ErrorCopyStatus").Visibility(Visibility::Collapsed);
         notice.Severity(InfoBarSeverity::Error);
         notice.Margin(Thickness{0,0,0,16});
         notice.IsOpen(true);
@@ -281,6 +283,15 @@ struct DesktopApp : ApplicationT<DesktopApp, Markup::IXamlMetadataProvider> {
         clicked(L"ViewDownloadsButton", [this] { select_page(2); });
         clicked(L"DownloadsFindButton", [this] { select_source(true); select_page(1); });
         clicked(L"CopyEndpointButton",[this]{if(endpoint_.empty())return;copy_text(endpoint_+L"/v1");copied_endpoint_=endpoint_;endpoint_copy_until_=std::chrono::steady_clock::now()+std::chrono::seconds(2);text(L"EndpointCopyStatus",L"Copied");});
+        clicked(L"CopyErrorButton",[this]{
+            try {
+                copy_text(std::wstring(control<TextBlock>(L"NoticeMessage").Text()));
+                text(L"ErrorCopyStatus",L"Copied");
+            } catch(hresult_error const&) {
+                text(L"ErrorCopyStatus",L"Clipboard unavailable. Select the message and press Ctrl+C.");
+            }
+            control<TextBlock>(L"ErrorCopyStatus").Visibility(Visibility::Visible);
+        });
         initialize_chat();
         clicked(L"RetryModelButton", [this] { if (!pending_model_id_.empty()) activate(pending_model_id_, pending_model_name_); });
         for (auto name : { L"StorageButton", L"DownloadStorageButton" }) clicked(name, [this] { storage_dialog(); });