fix: add O_BINARY flag to os.open for Windows attachment writes

Fixes #494 - Windows attachment corruption for PNG/PDF files.

On Windows, os.open() defaults to text mode, which translates LF (0x0a)
bytes to CRLF (0x0d 0x0a) during os.write(). This corrupts any binary
attachment containing 0x0a bytes (PNG headers, PDFs, etc.).

The fix adds os.O_BINARY to the os.open() flags using the standard
getattr(os, 'O_BINARY', 0) idiom, which returns the flag on Windows
and 0 (no-op) on platforms where it doesn't exist.

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

core/attachment_storage.py

@@ -102,7 +102,7 @@ class AttachmentStorage:
         # Save file with restrictive permissions (sensitive email/drive content)
         file_path = STORAGE_DIR / save_name
         try:
-            fd = os.open(file_path, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
+            fd = os.open(file_path, os.O_WRONLY | os.O_CREAT | os.O_TRUNC | getattr(os, 'O_BINARY', 0), 0o600)
             try:
                 total_written = 0
                 data_len = len(file_bytes)