feat(postit): add live capture previews on Android and Desktop

Reuse oriented Android EGL frames and duplicate Desktop video output before encoding without microphone monitoring. Cover preview lifecycle, rendering and recording with targeted tests and document the feature.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/PostIt/PostIt.Tests/PodcastBroadcasterTests.cs

@@ -1,7 +1,10 @@
 using System.Net;
 using Avalonia.Headless.XUnit;
+using Avalonia.Headless;
+using Avalonia.Controls;
 using PostIt.Services;
 using PostIt.ViewModels;
+using PostIt.Views;
 using Yavsc.Abstract.Streaming;
 using Yavsc.Api.Client;
 
@@ -9,6 +12,69 @@ namespace PostIt.Tests;
 
 public sealed class PodcastBroadcasterTests
 {
+    [Fact]
+    public Task Shared_page_exposes_running_capture_preview_and_clears_it_before_disposal()
+        => HeadlessUnitTestSession.GetOrStartForAssembly(typeof(PodcastBroadcasterTests).Assembly).Dispatch(async () =>
+        {
+            var tokenPath = Path.Combine(Path.GetTempPath(), $"postit-preview-{Guid.NewGuid():N}.json");
+            var devices = Platform.GetPodcastCaptureDevices;
+            var permissions = Platform.RequestPodcastPermissionsAsync;
+            var factory = Platform.CreatePodcastCaptureAsync;
+            var store = new TokenStore(tokenPath);
+            store.Save(new RefreshTokenRecord("test-access-token", "test-refresh-token", DateTimeOffset.UtcNow.AddHours(1), null));
+            await using var api = new YavscApiClient(new Settings(), store);
+            await using var transport = new Transport();
+            FakeCapture? capture = null;
+            PodcastsPageViewModel? vm = null;
+            try
+            {
+                var preview = new Border();
+                var disposed = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+                Platform.GetPodcastCaptureDevices = () => new([new("Camera", "camera")], [new("None", "")]);
+                Platform.RequestPodcastPermissionsAsync = (_, _) => Task.CompletedTask;
+                Platform.CreatePodcastCaptureAsync = (_, _, directory, _) =>
+                {
+                    capture = new FakeCapture(directory, preview)
+                    {
+                        OnDispose = () =>
+                        {
+                            Assert.Null(vm!.CapturePreview);
+                            disposed.TrySetResult();
+                        }
+                    };
+                    return Task.FromResult<IPodcastCapture>(capture);
+                };
+                var session = new SessionStatusViewModel { Api = api };
+                session.Refresh();
+                vm = new PodcastsPageViewModel(new PodcastApiClient(transport, () => "https://api.example/"), api, session)
+                {
+                    Title = "Preview"
+                };
+                vm.DetectDevicesCommand.Execute(null);
+                await vm.StartCommand.ExecuteAsync(null);
+                Assert.Same(preview, vm.CapturePreview);
+                Assert.True(vm.IsBroadcasting);
+                var page = new PodcastsPage { DataContext = vm };
+                var host = page.FindControl<ContentControl>("CapturePreviewHost");
+                Assert.NotNull(host);
+                Assert.Same(preview, host.Content);
+                await vm.DeactivateAsync();
+                await disposed.Task.WaitAsync(TimeSpan.FromSeconds(5), TestContext.Current.CancellationToken);
+                Assert.Null(vm.CapturePreview);
+                Assert.Null(host.Content);
+                Assert.True(capture!.Stopped);
+                return true;
+            }
+            finally
+            {
+                if (vm?.IsBroadcasting == true) await vm.StopAsync();
+                Platform.GetPodcastCaptureDevices = devices;
+                Platform.RequestPodcastPermissionsAsync = permissions;
+                Platform.CreatePodcastCaptureAsync = factory;
+                File.Delete(tokenPath);
+            }
+        }, TestContext.Current.CancellationToken);
+
     [AvaloniaFact]
     public async Task Shared_page_uses_platform_devices_and_requests_permissions_before_creating_server_broadcast()
     {
@@ -120,9 +186,12 @@ public sealed class PodcastBroadcasterTests
         public string Directory { get; }
         public bool Stopped { get; private set; }
         public bool Disposed { get; private set; }
+        public Control? Preview { get; }
+        public Action? OnDispose { get; init; }
         public Task Completion => _completion.Task;
-        public FakeCapture(string directory)
+        public FakeCapture(string directory, Control? preview = null)
         {
+            Preview = preview;
             Directory = directory;
             File.WriteAllBytes(Path.Combine(directory, "segment-00000000.ts"), [0x47, 1]);
             File.WriteAllText(Path.Combine(directory, "index.m3u8"), "#EXTM3U\n#EXTINF:2,\nsegment-00000000.ts\n");
@@ -136,7 +205,7 @@ public sealed class PodcastBroadcasterTests
             _completion.TrySetResult();
             return Task.CompletedTask;
         }
-        public ValueTask DisposeAsync() { Disposed = true; return ValueTask.CompletedTask; }
+        public ValueTask DisposeAsync() { Disposed = true; OnDispose?.Invoke(); return ValueTask.CompletedTask; }
     }
 
     private sealed class Transport : IYavscApiClient