Fix IPC stdout corruption, dark window background, overlay timing

- Redirect sys.stdout to stderr in Python sidecar so library print()
  calls don't corrupt the JSON-line IPC stream
- Save real stdout fd for exclusive IPC use via init_ipc()
- Skip non-JSON lines in Rust reader instead of failing with parse error
- Set Tauri window background color to match dark theme (#0a0a23)
- Add inline dark background on html/body to prevent white flash
- Use Svelte tick() to ensure progress overlay renders before invoke
- Improve ProgressOverlay with spinner, better styling, z-index 9999

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

python/tests/test_protocol.py

@@ -5,16 +5,23 @@ import json
 
 from voice_to_notes.ipc.messages import IPCMessage
 from voice_to_notes.ipc.protocol import read_message, write_message
+import voice_to_notes.ipc.protocol as protocol
 
 
-def test_write_message(capsys):
-    msg = IPCMessage(id="req-1", type="pong", payload={"ok": True})
-    write_message(msg)
-    captured = capsys.readouterr()
-    parsed = json.loads(captured.out.strip())
-    assert parsed["id"] == "req-1"
-    assert parsed["type"] == "pong"
-    assert parsed["payload"]["ok"] is True
+def test_write_message():
+    buf = io.StringIO()
+    # Temporarily replace the IPC output stream
+    old_out = protocol._ipc_out
+    protocol._ipc_out = buf
+    try:
+        msg = IPCMessage(id="req-1", type="pong", payload={"ok": True})
+        write_message(msg)
+        parsed = json.loads(buf.getvalue().strip())
+        assert parsed["id"] == "req-1"
+        assert parsed["type"] == "pong"
+        assert parsed["payload"]["ok"] is True
+    finally:
+        protocol._ipc_out = old_out
 
 
 def test_read_message(monkeypatch):