feat: surface app version and update awareness

src/SharpFM.Plugin/IUpdateCheckable.cs

@@ -0,0 +1,29 @@
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace SharpFM.Plugin;
+
+/// <summary>
+/// Opt-in capability for plugins that have a remote update channel. Implementing
+/// this interface is independent of <see cref="IPlugin"/> — a plugin opts in by
+/// implementing both. Plugins that ship with the host binary (or otherwise have
+/// no separate release cadence) should not implement this; the host will simply
+/// display <see cref="IPlugin.Version"/> without a check-for-updates affordance.
+/// </summary>
+public interface IUpdateCheckable
+{
+    /// <summary>
+    /// Query the plugin's update channel and return what the host should surface
+    /// in the About dialog. Implementations choose the channel (public manifest,
+    /// licensing endpoint, anonymous GitHub releases for a public repo, etc.) —
+    /// the host has no knowledge of any specific plugin's source.
+    ///
+    /// <para>
+    /// Implementations should fail silently on network errors, returning a
+    /// result with <see cref="UpdateCheckResult.UpdateAvailable"/> set to
+    /// <c>false</c> rather than throwing. A thrown exception is treated by the
+    /// host as a check failure and silently suppressed.
+    /// </para>
+    /// </summary>
+    Task<UpdateCheckResult> CheckForUpdatesAsync(CancellationToken ct);
+}

src/SharpFM.Plugin/UpdateCheckResult.cs

@@ -0,0 +1,30 @@
+using System;
+
+namespace SharpFM.Plugin;
+
+/// <summary>
+/// Outcome of an <see cref="IUpdateCheckable.CheckForUpdatesAsync"/> call.
+/// All three optional fields may be <c>null</c> when
+/// <see cref="UpdateAvailable"/> is <c>false</c>.
+/// </summary>
+/// <param name="UpdateAvailable">
+/// <c>true</c> when the channel reports a newer version than what the plugin
+/// is currently running.
+/// </param>
+/// <param name="LatestVersion">
+/// Human-readable version string of the latest release (e.g. <c>"2.0.0"</c>).
+/// Format is the plugin's choice; the host displays it verbatim.
+/// </param>
+/// <param name="ReleaseUrl">
+/// Where to point the user to obtain the update — typically a release notes
+/// or download page. Opened in the system browser on click.
+/// </param>
+/// <param name="Notes">
+/// Optional short summary surfaced alongside the version (e.g. release
+/// highlights). The host shows it as plain text; do not include markup.
+/// </param>
+public sealed record UpdateCheckResult(
+    bool UpdateAvailable,
+    string? LatestVersion,
+    Uri? ReleaseUrl,
+    string? Notes);

src/SharpFM/Dialogs/AboutDialog.axaml

@@ -0,0 +1,102 @@
+<Window
+    x:Class="SharpFM.Dialogs.AboutDialog"
+    xmlns="https://github.com/avaloniaui"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:vm="using:SharpFM.ViewModels"
+    Title="About SharpFM"
+    Width="480"
+    x:DataType="vm:AboutViewModel"
+    CanResize="False"
+    SizeToContent="Height"
+    WindowStartupLocation="CenterOwner">
+
+    <Grid Margin="20" RowDefinitions="Auto,Auto,Auto,Auto,Auto,*,Auto">
+
+        <TextBlock
+            Grid.Row="0"
+            Classes="Fluent2Title"
+            Text="{Binding Host.DisplayName}" />
+
+        <TextBlock
+            Grid.Row="1"
+            Margin="0,0,0,12"
+            Classes="Fluent2Body"
+            Opacity="0.7"
+            Text="{Binding Host.Version}" />
+
+        <StackPanel
+            Grid.Row="2"
+            Margin="0,0,0,4"
+            Orientation="Horizontal"
+            Spacing="8">
+            <Button
+                x:Name="hostCheckButton"
+                Classes="Fluent2"
+                Content="Check for updates" />
+            <Button
+                x:Name="hostOpenUpdateButton"
+                Classes="Fluent2"
+                Content="View release"
+                IsVisible="False" />
+            <Button
+                x:Name="hostHomepageButton"
+                Classes="Fluent2"
+                Content="GitHub" />
+        </StackPanel>
+
+        <TextBlock
+            Grid.Row="3"
+            Margin="0,0,0,12"
+            Classes="Fluent2Caption"
+            IsVisible="{Binding Host.Status, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
+            Text="{Binding Host.Status}" />
+
+        <TextBlock
+            Grid.Row="4"
+            Margin="0,8,0,8"
+            Classes="Fluent2Subtitle"
+            IsVisible="{Binding Plugins.Count}"
+            Text="Loaded plugins" />
+
+        <ItemsControl Grid.Row="5" ItemsSource="{Binding Plugins}">
+            <ItemsControl.ItemTemplate>
+                <DataTemplate x:DataType="vm:AboutEntryViewModel">
+                    <Grid Margin="0,4" ColumnDefinitions="*,Auto">
+                        <StackPanel Grid.Column="0" Spacing="2">
+                            <TextBlock
+                                Classes="Fluent2Body"
+                                FontWeight="SemiBold"
+                                Text="{Binding DisplayName}" />
+                            <TextBlock
+                                Classes="Fluent2Caption"
+                                Opacity="0.7"
+                                Text="{Binding Version}" />
+                            <TextBlock
+                                Classes="Fluent2Caption"
+                                IsVisible="{Binding Status, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
+                                Opacity="0.7"
+                                Text="{Binding Status}" />
+                        </StackPanel>
+                        <Button
+                            Grid.Column="1"
+                            Classes="Fluent2"
+                            Click="OnPluginCheckClick"
+                            Content="Check for updates"
+                            IsVisible="{Binding CanCheckForUpdates}"
+                            Tag="{Binding}" />
+                    </Grid>
+                </DataTemplate>
+            </ItemsControl.ItemTemplate>
+        </ItemsControl>
+
+        <Button
+            x:Name="closeButton"
+            Grid.Row="6"
+            Margin="0,16,0,0"
+            HorizontalAlignment="Right"
+            Classes="Fluent2Primary"
+            Content="Close" />
+
+    </Grid>
+
+</Window>

src/SharpFM/Dialogs/AboutDialog.axaml.cs

@@ -0,0 +1,78 @@
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+using Avalonia.Markup.Xaml;
+using SharpFM.ViewModels;
+
+namespace SharpFM.Dialogs;
+
+/// <summary>
+/// Help → About window. Surfaces host version and the list of loaded plugins.
+/// Each entry that opted in via <c>IUpdateCheckable</c> gets a check-for-updates
+/// affordance; checks run against whatever channel the implementer chose.
+/// </summary>
+[ExcludeFromCodeCoverage]
+public partial class AboutDialog : Window
+{
+    private AboutViewModel? _vm;
+
+    public AboutDialog()
+    {
+        AvaloniaXamlLoader.Load(this);
+
+        this.FindControl<Button>("closeButton")!.Click += (_, _) => Close();
+        this.FindControl<Button>("hostCheckButton")!.Click += OnHostCheckClick;
+        this.FindControl<Button>("hostOpenUpdateButton")!.Click += OnHostOpenUpdateClick;
+        this.FindControl<Button>("hostHomepageButton")!.Click += OnHostHomepageClick;
+    }
+
+    public void Configure(AboutViewModel vm)
+    {
+        _vm = vm;
+        DataContext = vm;
+        vm.Host.PropertyChanged += (_, e) =>
+        {
+            if (e.PropertyName == nameof(AboutEntryViewModel.UpdateUrl))
+                this.FindControl<Button>("hostOpenUpdateButton")!.IsVisible = vm.Host.UpdateUrl is not null;
+        };
+    }
+
+    private async void OnHostCheckClick(object? sender, RoutedEventArgs e)
+    {
+        if (_vm is null) return;
+        await _vm.Host.CheckAsync(CancellationToken.None);
+    }
+
+    private void OnHostOpenUpdateClick(object? sender, RoutedEventArgs e)
+    {
+        if (_vm?.Host.UpdateUrl is { } url) OpenInBrowser(url.ToString());
+    }
+
+    private void OnHostHomepageClick(object? sender, RoutedEventArgs e)
+    {
+        if (_vm is null) return;
+        OpenInBrowser(_vm.HostHomepageUrl.ToString());
+    }
+
+    private async void OnPluginCheckClick(object? sender, RoutedEventArgs e)
+    {
+        if (sender is Button { Tag: AboutEntryViewModel entry })
+        {
+            await entry.CheckAsync(CancellationToken.None);
+        }
+    }
+
+    private static void OpenInBrowser(string url)
+    {
+        try
+        {
+            Process.Start(new ProcessStartInfo(url) { UseShellExecute = true });
+        }
+        catch
+        {
+            // Browser unavailable / sandboxed — silently no-op.
+        }
+    }
+}

src/SharpFM/MainWindow.axaml

@@ -76,6 +76,9 @@
                 <!--  Plugin menu items are added dynamically in code-behind  -->
                 <MenuItem x:Name="managePluginsMenuItem" Header="Manage Plugins..." />
             </MenuItem>
+            <MenuItem Header="_Help">
+                <MenuItem x:Name="aboutMenuItem" Header="About SharpFM..." />
+            </MenuItem>
         </Menu>
 
         <!--  Status bar  -->

src/SharpFM/MainWindow.axaml.cs

@@ -9,6 +9,7 @@
 using AvaloniaEdit;
 using AvaloniaEdit.Editing;
 using SharpFM.Diagnostics;
+using SharpFM.Dialogs;
 using SharpFM.Plugin;
 using SharpFM.Plugin.UI;
 using SharpFM.PluginManager;
@@ -38,6 +39,11 @@ public MainWindow()
         if (rawClipboard != null)
             rawClipboard.Click += (_, _) => new RawClipboardWindow().Show(this);
 
+        // "About SharpFM..." menu item
+        var about = this.FindControl<MenuItem>("aboutMenuItem");
+        if (about != null)
+            about.Click += (_, _) => ShowAbout();
+
         // Tunnel-phase Ctrl+V — see OnPreviewKeyDown.
         AddHandler(KeyDownEvent, OnPreviewKeyDown, RoutingStrategies.Tunnel);
 
@@ -202,6 +208,31 @@ private void ShowPluginManager()
         window.ShowDialog(this);
     }
 
+    // Shared across About-dialog opens so we don't churn sockets. The dialog
+    // can't open often enough for the static lifetime to matter; this just
+    // keeps GitHub's per-IP rate-limit accounting consistent across opens.
+    private static readonly System.Net.Http.HttpClient AboutHttpClient = new()
+    {
+        Timeout = TimeSpan.FromSeconds(10),
+    };
+
+    private void ShowAbout()
+    {
+        if (DataContext is not MainWindowViewModel vm) return;
+
+        var hostChecker = new HostUpdateCheck(AboutHttpClient, MainWindowViewModel.Version);
+        var aboutVm = new AboutViewModel(
+            "SharpFM",
+            MainWindowViewModel.Version,
+            hostChecker,
+            new Uri("https://github.com/fuzzzerd/SharpFM"),
+            vm.AllPlugins);
+
+        var dialog = new AboutDialog();
+        dialog.Configure(aboutVm);
+        dialog.ShowDialog(this);
+    }
+
     // --- Tree / tab interaction ---
 
     // Walk the visual tree up from the tapped item to find the clip node that