fix(v3-actions): fix UnicodeEncodeError in download filenames

Hardened StreamingResponse headers to use RFC 6266 filename* parameter with UTF-8 encoding. This prevents the latin-1 codec crash when filenames contain non-ASCII characters like smart quotes.

app/routers/api_v3_actions_hosted_file.py

@@ -7,6 +7,7 @@ import pathlib
 from typing import Dict, List, Optional, Set, Union
 import asyncio
 import logging
+from urllib.parse import quote
 
 log = logging.getLogger(__name__)
 
@@ -286,6 +287,12 @@ async def download_file_action(
         end = min(end, file_size - 1)
         content_length = end - start + 1
 
+        # ID Vision: Properly encode filename for headers to avoid UnicodeEncodeError (latin-1)
+        # 1. Standard filename (Sanitized for legacy clients - latin-1 safe)
+        safe_filename = target_filename.encode('ascii', errors='ignore').decode('ascii')
+        # 2. filename* (UTF-8 encoded for modern clients)
+        encoded_filename = quote(target_filename)
+
         return StreamingResponse(
             file_streamer(full_file_path, start, end + 1),
             media_type = media_type,
@@ -294,7 +301,7 @@ async def download_file_action(
                 'Accept-Ranges': 'bytes',
                 'Content-Range': f'bytes {start}-{end}/{file_size}',
                 'Content-Length': str(content_length),
-                'Content-Disposition': f'attachment; filename="{target_filename}"'
+                'Content-Disposition': f'attachment; filename="{safe_filename}"; filename*=utf-8\'\'{encoded_filename}'
             }
         )