feat(voice-client): PySide6 desktop client and Windows build scripts

Add voice_confirmation_client (poll, TTS MP3 playback, mic WAV resolve),
PyInstaller spec, start/build helpers, and API unit tests.

Pending manual testing: end-to-end on OR workstations and packaged exe.

Made-with: Cursor

voice_confirmation_client/core/playback.py

@@ -0,0 +1,61 @@
+"""Play MP3 bytes via system player or bundled ffplay."""
+
+from __future__ import annotations
+
+import base64
+import os
+import shutil
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+
+from voice_confirmation_client.core.paths import find_ffplay
+
+
+def play_mp3_from_base64(b64: str) -> None:
+    raw_b64 = "".join((b64 or "").split())
+    if not raw_b64:
+        raise ValueError("empty prompt_audio_mp3_base64")
+    data = base64.b64decode(raw_b64, validate=False)
+    with tempfile.NamedTemporaryFile(suffix=".mp3", delete=False) as f:
+        f.write(data)
+        tmp = f.name
+    try:
+        _play_mp3_path(Path(tmp))
+    finally:
+        try:
+            os.unlink(tmp)
+        except OSError:
+            pass
+
+
+def _play_mp3_path(path: Path) -> None:
+    bundled = find_ffplay()
+    if bundled and bundled.is_file():
+        subprocess.run(
+            [str(bundled), "-nodisp", "-autoexit", "-loglevel", "quiet", str(path)],
+            check=True,
+            timeout=600,
+        )
+        return
+    ffplay = shutil.which("ffplay")
+    if ffplay:
+        subprocess.run(
+            [ffplay, "-nodisp", "-autoexit", "-loglevel", "quiet", str(path)],
+            check=True,
+            timeout=600,
+        )
+        return
+    if sys.platform == "darwin":
+        subprocess.run(["afplay", str(path)], check=True, timeout=600)
+        return
+    if os.name == "nt":
+        os.startfile(str(path))  # type: ignore[attr-defined]
+        import time
+
+        time.sleep(5)
+        return
+    raise RuntimeError(
+        "No MP3 player found. Install ffmpeg (ffplay) or run on macOS with afplay."
+    )