feat: cross-platform rewrite with Avalonia UI + Linux support

App.Avalonia/App.Avalonia.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>WinExe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <RootNamespace>Coder.Desktop.App</RootNamespace>
+    <AssemblyName>Coder Desktop</AssemblyName>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>preview</LangVersion>
+    <BuiltInComInteropSupport>true</BuiltInComInteropSupport>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Avalonia" Version="11.2.3" />
+    <PackageReference Include="Avalonia.Desktop" Version="11.2.3" />
+    <PackageReference Include="Avalonia.Themes.Fluent" Version="11.2.3" />
+    <PackageReference Include="Avalonia.Fonts.Inter" Version="11.2.3" />
+    <PackageReference Include="Avalonia.Diagnostics" Version="11.2.3" Condition="'$(Configuration)' == 'Debug'" />
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="9.0.9" />
+    <PackageReference Include="Microsoft.Extensions.Hosting" Version="9.0.9" />
+    <PackageReference Include="Serilog.Extensions.Hosting" Version="9.0.0" />
+    <PackageReference Include="Serilog.Sinks.File" Version="7.0.0" />
+    <PackageReference Include="Serilog.Sinks.Console" Version="6.0.0" />
+  </ItemGroup>
+
+  <ItemGroup Condition="$([MSBuild]::IsOSPlatform('Windows'))">
+    <ProjectReference Include="..\Vpn.Windows\Vpn.Windows.csproj" />
+  </ItemGroup>
+  <ItemGroup Condition="$([MSBuild]::IsOSPlatform('Linux'))">
+    <ProjectReference Include="..\App.Linux\App.Linux.csproj" />
+    <ProjectReference Include="..\Vpn.Linux\Vpn.Linux.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\App.Shared\App.Shared.csproj" Aliases="global,AppShared" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <AvaloniaResource Include="Assets\\**" />
+  </ItemGroup>
+</Project>

App.Avalonia/App.axaml

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+<Application xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:converters="clr-namespace:Coder.Desktop.App.Converters"
+             x:Class="Coder.Desktop.App.App"
+             RequestedThemeVariant="Default">
+
+    <!--
+        Note: pages are implemented in parallel tasks (T5/T6). We register
+        converters here so future pages can reference them via StaticResource.
+    -->
+    <Application.Resources>
+        <converters:InverseBoolConverter x:Key="InverseBoolConverter" />
+        <converters:BoolToObjectConverter x:Key="BoolToObjectConverter" />
+        <converters:FriendlyByteConverter x:Key="FriendlyByteConverter" />
+        <converters:VpnLifecycleToBoolConverter x:Key="VpnLifecycleToBoolConverter" />
+        <converters:StringToBrushSelector x:Key="StringToBrushSelector" />
+        <converters:StringToStringSelector x:Key="StringToStringSelector" />
+    </Application.Resources>
+
+    <Application.Styles>
+        <FluentTheme />
+    </Application.Styles>
+
+    <TrayIcon.Icons>
+        <TrayIcons>
+            <TrayIcon Icon="/Assets/coder.ico"
+                      ToolTipText="Coder Desktop"
+                      Command="{Binding ShowWindowCommand}">
+                <TrayIcon.Menu>
+                    <NativeMenu>
+                        <NativeMenuItem Header="Show" Command="{Binding ShowWindowCommand}" />
+                        <NativeMenuItemSeparator />
+                        <NativeMenuItem Header="Check for Updates" />
+                        <NativeMenuItemSeparator />
+                        <NativeMenuItem Header="Exit" Command="{Binding ExitCommand}" />
+                    </NativeMenu>
+                </TrayIcon.Menu>
+            </TrayIcon>
+        </TrayIcons>
+    </TrayIcon.Icons>
+</Application>

App.Avalonia/App.axaml.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Markup.Xaml;
+using Coder.Desktop.App.Services;
+using Coder.Desktop.App.ViewModels;
+using Coder.Desktop.App.Views;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Coder.Desktop.App;
+
+public partial class App : Application
+{
+    private ServiceProvider? _services;
+    private TrayWindow? _trayWindow;
+    private TrayIconViewModel? _trayIconViewModel;
+
+    public override void Initialize()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+
+    public override void OnFrameworkInitializationCompleted()
+    {
+        var services = new ServiceCollection();
+
+        // Register cross-platform services from App.Shared
+        services.AddSingleton<IDispatcher, AvaloniaDispatcher>();
+        services.AddSingleton<IClipboardService, AvaloniaClipboardService>();
+        services.AddSingleton<ILauncherService, ProcessLauncherService>();
+
+        _services = services.BuildServiceProvider();
+
+        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
+            desktop.ShutdownMode = ShutdownMode.OnExplicitShutdown;
+
+            // Create the tray popup window immediately so we can show/hide it
+            // quickly from the tray icon.
+            _trayWindow = new TrayWindow();
+
+            // Make TrayWindow the MainWindow so dialogs can use it as an owner.
+            desktop.MainWindow = _trayWindow;
+
+            // Keep it hidden on startup; the user will open it from the tray.
+            // (StartWithClassicDesktopLifetime will show MainWindow by default.)
+            _trayWindow.RequestHideOnFirstOpen();
+
+            _trayIconViewModel = new TrayIconViewModel(ToggleTrayWindow, () => desktop.Shutdown());
+            ConfigureTrayIcons(_trayIconViewModel);
+        }
+
+        base.OnFrameworkInitializationCompleted();
+    }
+
+    private void ConfigureTrayIcons(TrayIconViewModel trayIconViewModel)
+    {
+        // The tray icons are defined in App.axaml via the TrayIcon.Icons attached property.
+        var icons = TrayIcon.GetIcons(this);
+        if (icons is null)
+            return;
+
+        foreach (var trayIcon in icons)
+        {
+            // Ensure clicking the icon toggles the tray window.
+            trayIcon.Clicked -= TrayIconOnClicked;
+            trayIcon.Clicked += TrayIconOnClicked;
+
+            // Also set the command explicitly so bindings don't depend on a DataContext
+            // being available for tray icon objects.
+            trayIcon.Command = trayIconViewModel.ShowWindowCommand;
+
+            if (trayIcon.Menu is NativeMenu menu)
+            {
+                foreach (var item in menu.Items)
+                {
+                    if (item is not NativeMenuItem nativeItem)
+                        continue;
+
+                    switch (nativeItem.Header?.ToString())
+                    {
+                        case "Show":
+                            nativeItem.Command = trayIconViewModel.ShowWindowCommand;
+                            break;
+                        case "Exit":
+                            nativeItem.Command = trayIconViewModel.ExitCommand;
+                            break;
+                    }
+                }
+            }
+        }
+    }
+
+    private void TrayIconOnClicked(object? sender, EventArgs e)
+    {
+        ToggleTrayWindow();
+    }
+
+    private void ToggleTrayWindow()
+    {
+        if (_trayWindow is null)
+            return;
+
+        if (_trayWindow.IsVisible)
+        {
+            _trayWindow.Hide();
+            return;
+        }
+
+        _trayWindow.ShowNearSystemTray();
+    }
+}

App.Avalonia/Controls/ExpandChevron.axaml

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+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             x:Class="Coder.Desktop.App.Controls.ExpandChevron">
+
+    <Grid>
+        <PathIcon x:Name="ChevronIcon"
+                  Width="16"
+                  Height="16"
+                  Margin="0,0,8,0"
+                  RenderTransformOrigin="0.5,0.5"
+                  HorizontalAlignment="Center"
+                  VerticalAlignment="Center"
+                  Foreground="{Binding Foreground, RelativeSource={RelativeSource AncestorType=UserControl}}"
+                  Data="M 6,3 L 10.5,8 L 6,13 L 7.4,14.4 L 13.8,8 L 7.4,1.6 Z">
+
+            <PathIcon.RenderTransform>
+                <RotateTransform Angle="0">
+                    <RotateTransform.Transitions>
+                        <Transitions>
+                            <DoubleTransition Property="Angle" Duration="0:0:0.15" />
+                        </Transitions>
+                    </RotateTransform.Transitions>
+                </RotateTransform>
+            </PathIcon.RenderTransform>
+        </PathIcon>
+    </Grid>
+</UserControl>

App.Avalonia/Controls/ExpandChevron.axaml.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Media;
+
+namespace Coder.Desktop.App.Controls;
+
+public partial class ExpandChevron : UserControl
+{
+    public static readonly StyledProperty<bool> IsOpenProperty =
+        AvaloniaProperty.Register<ExpandChevron, bool>(nameof(IsOpen));
+
+    static ExpandChevron()
+    {
+        IsOpenProperty.Changed.AddClassHandler<ExpandChevron>((x, e) =>
+        {
+            if (e.NewValue is bool isOpen)
+            {
+                x.UpdateChevronAngle(isOpen);
+            }
+        });
+    }
+
+    public bool IsOpen
+    {
+        get => GetValue(IsOpenProperty);
+        set => SetValue(IsOpenProperty, value);
+    }
+
+    public ExpandChevron()
+    {
+        InitializeComponent();
+
+        UpdateChevronAngle(IsOpen);
+    }
+
+    private void UpdateChevronAngle(bool isOpen)
+    {
+        if (ChevronIcon is null)
+        {
+            return;
+        }
+
+        if (ChevronIcon.RenderTransform is RotateTransform rotate)
+        {
+            rotate.Angle = isOpen ? 90 : 0;
+        }
+    }
+}

App.Avalonia/Controls/ExpandContent.axaml

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+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             x:Class="Coder.Desktop.App.Controls.ExpandContent">
+
+    <Grid x:Name="CollapsiblePanel"
+          Opacity="0"
+          IsVisible="False"
+          MaxHeight="0"
+          ClipToBounds="True">
+
+        <Grid.Transitions>
+            <Transitions>
+                <DoubleTransition Property="MaxHeight" Duration="0:0:0.16" />
+                <DoubleTransition Property="Opacity" Duration="0:0:0.16" />
+            </Transitions>
+        </Grid.Transitions>
+
+        <Grid.RenderTransform>
+            <TranslateTransform Y="-16">
+                <TranslateTransform.Transitions>
+                    <Transitions>
+                        <DoubleTransition Property="Y" Duration="0:0:0.16" />
+                    </Transitions>
+                </TranslateTransform.Transitions>
+            </TranslateTransform>
+        </Grid.RenderTransform>
+    </Grid>
+</UserControl>