feat: add audio playback and file download features with corresponding UI updates

src/EchoHub.Client/Services/ApiClient.cs

@@ -196,6 +196,23 @@ public sealed class ApiClient : IDisposable
         return await response.Content.ReadFromJsonAsync<MessageDto>();
     }
 
+    public async Task<string> DownloadFileToTempAsync(string relativeUrl, string fileName)
+    {
+        EnsureAuthenticated();
+        var response = await AuthenticatedGetAsync(relativeUrl);
+        await EnsureSuccessAsync(response);
+
+        var tempDir = Path.Combine(Path.GetTempPath(), "EchoHub");
+        Directory.CreateDirectory(tempDir);
+        var tempPath = Path.Combine(tempDir, $"{Guid.NewGuid():N}_{fileName}");
+
+        await using var stream = await response.Content.ReadAsStreamAsync();
+        await using var file = File.Create(tempPath);
+        await stream.CopyToAsync(file);
+
+        return tempPath;
+    }
+
     public async Task<ChannelDto?> CreateChannelAsync(string name, string? topic = null, bool isPublic = true)
     {
         EnsureAuthenticated();

src/EchoHub.Client/Services/AudioPlaybackService.cs

@@ -0,0 +1,39 @@
+using NetCoreAudio;
+using Serilog;
+
+namespace EchoHub.Client.Services;
+
+public class AudioPlaybackService
+{
+    private readonly Player _player = new();
+
+    public bool IsPlaying => _player.Playing;
+
+    public async Task PlayAsync(string filePath)
+    {
+        try
+        {
+            if (_player.Playing)
+                await _player.Stop();
+
+            await _player.Play(filePath);
+        }
+        catch (Exception ex)
+        {
+            Log.Warning(ex, "Failed to play audio file: {Path}", filePath);
+        }
+    }
+
+    public async Task StopAsync()
+    {
+        try
+        {
+            if (_player.Playing)
+                await _player.Stop();
+        }
+        catch (Exception ex)
+        {
+            Log.Warning(ex, "Failed to stop audio playback");
+        }
+    }
+}