feat(mcp): Rust-owned speaking pill with self-contained audio playback

The pill window now surfaces for agent-initiated speech without main-window
involvement. Rust subscribes to /events/speak via a tokio task + reqwest
streaming body (speak_monitor.rs), shows the pill, and forwards events to
the dictate webview over Tauri's event bus. The pill plays audio via a
plain HTMLAudioElement and emits dictate:hide when playback ends. The
pill stays hidden through the ~1 s generation wait and only surfaces when
audio actually starts, with the counter armed at that moment.

Fixes a shared-dict mutation in mcp_server/events.publish() that caused
the second subscriber (Rust speak_monitor) to receive `event: message`
instead of named speak-start/speak-end frames. Also teaches the speak_monitor
parser to handle CRLF framing (sse-starlette default). Main-window
AudioPlayer now skips autoplay for source in {mcp, rest} to avoid
double-play when both windows are alive.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

backend/mcp_server/events.py

@@ -25,9 +25,15 @@ def unsubscribe(queue: asyncio.Queue[dict[str, Any]]) -> None:
 
 
 def publish(kind: str, payload: dict[str, Any]) -> None:
-    """Fan out to all current subscribers. Non-blocking; drops on full queue."""
-    event = {"kind": kind, **payload}
+    """Fan out to all current subscribers. Non-blocking; drops on full queue.
+
+    Each subscriber gets its own dict copy — the SSE consumer calls
+    ``event.pop("kind", ...)``, so sharing a single dict between queues
+    would mean the first consumer to drain its queue strips ``kind`` from
+    the object the next consumer later reads.
+    """
     for queue in list(_subscribers):
+        event = {"kind": kind, **payload}
         try:
             queue.put_nowait(event)
         except asyncio.QueueFull: