feat(android): share podcast playback and foreground broadcasting

Add native Camera2 capture, MPEG-TS segmentation and foreground service with notification stop. Reuse shared authenticated uploads and replay finalization, add capture regression tests and document platform limitations. Update Avalonia to 12.1.4.

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

src/PostIt/PostIt.Tests/AndroidPodcastCaptureTestActivity.cs

@@ -0,0 +1,100 @@
+#if ANDROID
+using System;
+using System.IO;
+using System.Runtime.Versioning;
+using System.Threading;
+using System.Threading.Tasks;
+using Android.App;
+using Android.Content;
+using Android.OS;
+using Avalonia.Threading;
+using PostIt.Android;
+using PostIt.Android.Services;
+using PostIt.Services;
+
+namespace PostIt.Tests;
+
+/// <summary>Debug-only local encoder test; never connects to the podcast API.</summary>
+[Activity(Name = "fr.pschneider.postit.PodcastCaptureTestActivity", Exported = true)]
+[SupportedOSPlatform("android26.0")]
+public sealed class AndroidPodcastCaptureTestActivity : Activity
+{
+    private bool _microphone = true;
+    private bool _notificationStop;
+    private int _cameraIndex;
+    protected override async void OnCreate(Bundle? state)
+    {
+        base.OnCreate(state);
+        _microphone = Intent?.GetStringExtra("microphone") != "false";
+        _notificationStop = Intent?.GetStringExtra("notificationStop") == "true";
+        _cameraIndex = Intent?.GetStringExtra("camera") == "front" ? 1 : 0;
+        var resultPath = Path.Combine(FilesDir!.AbsolutePath, "podcast-capture-test-result.txt");
+        File.WriteAllText(resultPath, "RUNNING");
+        var result = new Bundle();
+        try
+        {
+            await RunAsync(result);
+            File.WriteAllText(resultPath, $"PASS\nsegments={result.GetInt("segments")}\ndirectory={result.GetString("directory")}\nbackground={result.GetString("background")}");
+        }
+        catch (Exception ex)
+        {
+            File.WriteAllText(resultPath, $"FAIL\n{ex}");
+        }
+        finally { Finish(); }
+    }
+
+    private async Task RunAsync(Bundle result)
+    {
+        if (!OperatingSystem.IsAndroidVersionAtLeast(26))
+            throw new PlatformNotSupportedException("Android 8 required.");
+        var context = this;
+        var intent = new Intent(context, typeof(MainActivity)).AddFlags(ActivityFlags.NewTask);
+        StartActivity(intent);
+        for (var i = 0; i < 100 && !MainActivity.IsForeground; i++) await Task.Delay(100);
+        await Task.Delay(1000);
+        var directory = Path.Combine(context.FilesDir!.AbsolutePath, "podcast-capture-tests", Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(directory);
+        var devices = AndroidPodcastCapture.GetDevices();
+        if (devices.Cameras.Count == 0) throw new InvalidOperationException("No emulator camera.");
+        IPodcastCapture? capture = null;
+        try
+        {
+            capture = await Dispatcher.UIThread.InvokeAsync(() => AndroidPodcastCapture.StartAsync(
+                devices.Cameras[Math.Min(_cameraIndex, devices.Cameras.Count - 1)].Value,
+                _microphone ? "microphone" : null, directory, CancellationToken.None));
+            await Task.Delay(3000);
+            var before = SegmentCount(directory);
+            context.StartActivity(new Intent(Intent.ActionMain).AddCategory(Intent.CategoryHome).AddFlags(ActivityFlags.NewTask));
+            await Task.Delay(9000);
+            var after = SegmentCount(directory);
+            if (capture.Completion.IsFaulted) await capture.Completion;
+            if (after <= before) throw new InvalidOperationException($"Capture did not continue in background: {before} -> {after} segments.");
+            if (_notificationStop)
+            {
+                using var stop = PendingIntent.GetService(context, 1,
+                    new Intent(context, typeof(PodcastCaptureService)).SetAction(PodcastCaptureService.StopAction),
+                    PendingIntentFlags.NoCreate | PendingIntentFlags.Immutable)
+                    ?? throw new InvalidOperationException("Notification stop action missing.");
+                stop.Send();
+            }
+            else await capture.StopAsync().WaitAsync(TimeSpan.FromSeconds(15));
+            await capture.Completion.WaitAsync(TimeSpan.FromSeconds(15));
+            var segments = HlsCapturePlaylist.Parse(await File.ReadAllTextAsync(Path.Combine(directory, "index.m3u8")));
+            if (segments.Count < 3) throw new InvalidOperationException("Insufficient capture segments.");
+            result.PutInt("segments", segments.Count);
+            result.PutString("directory", directory);
+            result.PutString("background", $"segments {before} -> {after}");
+        }
+        finally
+        {
+            if (capture is not null) await capture.DisposeAsync();
+        }
+    }
+
+    private static int SegmentCount(string directory)
+    {
+        var path = Path.Combine(directory, "index.m3u8");
+        return File.Exists(path) ? HlsCapturePlaylist.Parse(File.ReadAllText(path)).Count : 0;
+    }
+}
+#endif