fix(preview): revert to FileResponse as default for all platforms (#975)

The previous commit (a19ddc14) restored Linux sendfile but kept the
manual streaming path for Windows via sys.platform guard. A Windows
user reports performance is still worse than v1.0.5.

Switch back to web.FileResponse for all files on all platforms as the
default. The IOCP crash is an edge case (fast scrolling through many
video previews) that affects few users, while the Python chunked I/O
performance penalty affects everyone.

_stream_file() is kept as an unused fallback for a future compat
setting toggle.

py/routes/handlers/preview_handlers.py

@@ -4,7 +4,6 @@ from __future__ import annotations
 
 import logging
 import mimetypes
-import sys
 import urllib.parse
 from pathlib import Path
 
@@ -56,16 +55,11 @@ class PreviewHandler:
             logger.debug("Preview file not found at %s", str(resolved))
             raise web.HTTPNotFound(text="Preview file not found")
 
-        # Video files on Windows: stream manually to avoid Windows IOCP native
-        # sendfile crash when the client disconnects mid-transfer (happens
-        # constantly when users scroll through a gallery of animated previews).
-        # On Linux/macOS, web.FileResponse uses kernel sendfile (zero-copy DMA)
-        # and does not have this issue, so it is safe and much faster.
-        suffix = resolved.suffix.lower()
-        if suffix in _VIDEO_EXTENSIONS and sys.platform == "win32":
-            return await self._stream_file(request, resolved)
-
-        # aiohttp's FileResponse handles range requests and content headers for us.
+        # aiohttp's FileResponse handles range requests, content headers, and
+        # uses kernel sendfile (zero-copy DMA) on Linux/macOS. On Windows it
+        # uses IOCP-based _sendfile_native which can crash when the client
+        # disconnects mid-transfer during fast scrolling. The _stream_file()
+        # fallback is kept for a future compat toggle.
         return web.FileResponse(path=resolved, chunk_size=_CHUNK_SIZE)
 
     async def _stream_file(