Add in-app documentation viewer page

Adds a Documentation footer page that fetches the docs/ folder from the
repository, renders markdown with the existing Markdown.Avalonia control,
and handles internal navigation, external links, and relative images.

- Fetch docs tree via new Refit IGitHubContentApi (GitHub git/trees)
- DocumentationService groups pages into humanized sections (README first),
  caches the tree listing and page markdown to disk (1 day TTL) and serves
  from cache on network failure
- DocumentationPathResolver: pure helpers for humanizing names and resolving
  relative .md links / image URLs (unit tested)
- DocumentationMarkdownViewer routes hyperlink clicks through a command
  (relative .md navigates in-app, http(s) opens in the browser) and rewrites
  relative image paths to raw URLs
- Loading, offline/error (with Retry), and "docs not available yet" states

Owner/repo/branch live as consts in DocumentationConstants (branch "main").

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: mohnjiles

StabilityMatrix.Avalonia/App.axaml.cs

@@ -464,7 +464,11 @@ internal static void ConfigurePageViewModels(IServiceCollection services)
                 provider.GetRequiredService<OutputsPageViewModel>(),
                 provider.GetRequiredService<WorkflowsPageViewModel>(),
             },
-            FooterPages = { provider.GetRequiredService<SettingsViewModel>() },
+            FooterPages =
+            {
+                provider.GetRequiredService<DocumentationViewModel>(),
+                provider.GetRequiredService<SettingsViewModel>(),
+            },
         });
     }
 
@@ -838,6 +842,15 @@ internal static IServiceCollection ConfigureServices(bool disableMessagePipeInte
             })
             .AddPolicyHandler(retryPolicy);
 
+        services
+            .AddRefitClient<IGitHubContentApi>(defaultRefitSettings)
+            .ConfigureHttpClient(c =>
+            {
+                c.BaseAddress = new Uri("https://api.github.com");
+                c.Timeout = TimeSpan.FromMinutes(1);
+            })
+            .AddPolicyHandler(retryPolicy);
+
         services
             .AddRefitClient<IHuggingFaceApi>(defaultRefitSettings) // Assuming defaultRefitSettings is suitable
             .ConfigureHttpClient(c =>

StabilityMatrix.Avalonia/Controls/DocumentationMarkdownViewer.cs

@@ -0,0 +1,80 @@
+using System;
+using System.Windows.Input;
+using Avalonia;
+using Markdown.Avalonia;
+
+namespace StabilityMatrix.Avalonia.Controls;
+
+/// <summary>
+/// A <see cref="BetterMarkdownScrollViewer"/> that routes hyperlink clicks through a
+/// bindable <see cref="LinkCommand"/> (so relative <c>.md</c> links can navigate in-app
+/// and external links can open in the browser) and resolves relative image paths against
+/// <see cref="ImageBaseUrl"/> via the engine's asset path root.
+/// </summary>
+public class DocumentationMarkdownViewer : BetterMarkdownScrollViewer
+{
+    /// <summary>
+    /// Command invoked when a hyperlink is clicked. The command parameter is the raw href string.
+    /// </summary>
+    public static readonly StyledProperty<ICommand?> LinkCommandProperty = AvaloniaProperty.Register<
+        DocumentationMarkdownViewer,
+        ICommand?
+    >(nameof(LinkCommand));
+
+    /// <summary>
+    /// Base URL used to resolve relative image paths in the rendered markdown
+    /// (e.g. the raw URL of the current page's folder).
+    /// </summary>
+    public static readonly StyledProperty<string?> ImageBaseUrlProperty = AvaloniaProperty.Register<
+        DocumentationMarkdownViewer,
+        string?
+    >(nameof(ImageBaseUrl));
+
+    public ICommand? LinkCommand
+    {
+        get => GetValue(LinkCommandProperty);
+        set => SetValue(LinkCommandProperty, value);
+    }
+
+    public string? ImageBaseUrl
+    {
+        get => GetValue(ImageBaseUrlProperty);
+        set => SetValue(ImageBaseUrlProperty, value);
+    }
+
+    public DocumentationMarkdownViewer()
+    {
+        ApplyLinkCommand();
+        ApplyImageBaseUrl();
+    }
+
+    protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+    {
+        base.OnPropertyChanged(change);
+
+        if (change.Property == LinkCommandProperty)
+        {
+            ApplyLinkCommand();
+        }
+        else if (change.Property == ImageBaseUrlProperty)
+        {
+            ApplyImageBaseUrl();
+        }
+    }
+
+    private void ApplyLinkCommand()
+    {
+        // The engine (IMarkdownEngine) owns the HyperlinkCommand used for all rendered links.
+        if (Engine is IMarkdownEngine engine)
+        {
+            engine.HyperlinkCommand = LinkCommand;
+        }
+    }
+
+    private void ApplyImageBaseUrl()
+    {
+        // AssetPathRoot flows through to the engine's bitmap loader so relative image
+        // paths resolve against the raw docs URL.
+        AssetPathRoot = ImageBaseUrl ?? string.Empty;
+    }
+}

StabilityMatrix.Avalonia/ViewModels/Documentation/DocumentationPageNavItem.cs

@@ -0,0 +1,18 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace StabilityMatrix.Avalonia.ViewModels.Documentation;
+
+/// <summary>
+/// A single navigable documentation page entry in the sidebar.
+/// </summary>
+public partial class DocumentationPageNavItem : ObservableObject
+{
+    /// <summary>Display title, e.g. "Overview".</summary>
+    public required string Title { get; init; }
+
+    /// <summary>Path relative to the docs root, e.g. <c>getting-started/overview.md</c>.</summary>
+    public required string Path { get; init; }
+
+    [ObservableProperty]
+    private bool isSelected;
+}

StabilityMatrix.Avalonia/ViewModels/Documentation/DocumentationSectionNavItem.cs

@@ -0,0 +1,18 @@
+using System.Collections.Generic;
+
+namespace StabilityMatrix.Avalonia.ViewModels.Documentation;
+
+/// <summary>
+/// A section grouping in the documentation sidebar (e.g. "Getting Started").
+/// </summary>
+public class DocumentationSectionNavItem
+{
+    /// <summary>Section title. Empty for the root section (renders without a header).</summary>
+    public required string Title { get; init; }
+
+    /// <summary>Whether this section has a visible header (i.e. is not the root section).</summary>
+    public bool HasHeader => !string.IsNullOrEmpty(Title);
+
+    /// <summary>Pages within this section.</summary>
+    public required IReadOnlyList<DocumentationPageNavItem> Pages { get; init; }
+}