Fix TTS Stop button never enabled during SSE streaming

The SSE streaming code path spoke sentences inline without setting
_isSpeaking=true, so UpdateButtons() never enabled the Stop button.
Now sets _isSpeaking before streaming and clears it after all TTS
completes.

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

Author: sharpninja

src/McpServerManager.Android/Views/SimplifiedVoiceView.axaml.cs

@@ -243,6 +243,9 @@ private async Task RunConversationLoopAsync(CancellationToken ct)
             var sentenceBuffer = new StringBuilder();
             var spokenUpTo = 0;
             var isDone = false;
+            _isSpeaking = true;
+            _ttsStopped = false;
+            UpdateButtons();
 
             await foreach (var evt in vm.SubmitTurnStreamingAsync(transcript, ct).ConfigureAwait(true))
             {
@@ -321,6 +324,9 @@ private async Task RunConversationLoopAsync(CancellationToken ct)
                 catch (OperationCanceledException) { /* ok */ }
             }
 
+            _isSpeaking = false;
+            UpdateButtons();
+
             ct.ThrowIfCancellationRequested();
             if (listenResult.ShouldEndChat)
             {