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/voice_to_notes/ipc/protocol.py

@@ -1,13 +1,53 @@
-"""JSON-line protocol reader/writer over stdin/stdout."""
+"""JSON-line protocol reader/writer over stdin/stdout.
+
+IMPORTANT: stdout is reserved exclusively for IPC messages.
+At init time we save the real stdout, then redirect sys.stdout → stderr
+so that any rogue print() calls from libraries don't corrupt the IPC stream.
+"""
 
 from __future__ import annotations
 
+import io
 import json
+import os
 import sys
 from typing import Any
 
 from voice_to_notes.ipc.messages import IPCMessage
 
+# Save the real stdout fd for IPC before any library can pollute it.
+# Then redirect sys.stdout to stderr so library prints go to stderr.
+_ipc_out: io.TextIOWrapper | None = None
+
+
+def init_ipc() -> None:
+    """Capture real stdout for IPC and redirect sys.stdout to stderr.
+
+    Must be called once at sidecar startup, before importing any ML libraries.
+    """
+    global _ipc_out
+    if _ipc_out is not None:
+        return  # already initialised
+
+    # Duplicate the real stdout fd so we keep it even after redirect
+    real_stdout_fd = os.dup(sys.stdout.fileno())
+    _ipc_out = io.TextIOWrapper(
+        io.BufferedWriter(io.FileIO(real_stdout_fd, "w")),
+        encoding="utf-8",
+        line_buffering=True,
+    )
+
+    # Redirect sys.stdout → stderr so print() from libraries goes to stderr
+    sys.stdout = sys.stderr
+
+
+def _get_ipc_out() -> io.TextIOWrapper:
+    """Return the IPC output stream, falling back to sys.__stdout__."""
+    if _ipc_out is not None:
+        return _ipc_out
+    # Fallback if init_ipc() was never called (e.g. in tests)
+    return sys.__stdout__
+
 
 def read_message() -> IPCMessage | None:
     """Read a single JSON-line message from stdin. Returns None on EOF."""
@@ -29,17 +69,19 @@ def read_message() -> IPCMessage | None:
 
 
 def write_message(msg: IPCMessage) -> None:
-    """Write a JSON-line message to stdout."""
+    """Write a JSON-line message to the IPC channel (real stdout)."""
+    out = _get_ipc_out()
     line = json.dumps(msg.to_dict(), separators=(",", ":"))
-    sys.stdout.write(line + "\n")
-    sys.stdout.flush()
+    out.write(line + "\n")
+    out.flush()
 
 
 def write_dict(data: dict[str, Any]) -> None:
-    """Write a raw dict as a JSON-line message to stdout."""
+    """Write a raw dict as a JSON-line message to the IPC channel."""
+    out = _get_ipc_out()
     line = json.dumps(data, separators=(",", ":"))
-    sys.stdout.write(line + "\n")
-    sys.stdout.flush()
+    out.write(line + "\n")
+    out.flush()
 
 
 def _log(message: str) -> None: