feat(browser): auto-snapshot on navigate, screenshot prompt guidance

Browser tool enhancements:
- Navigate action now auto-includes snapshot result, saving one LLM round-trip
- Wait for networkidle + 800ms after navigation for SPA/JS-rendered pages
- Prompt guides agent to screenshot key results and ask user for login/CAPTCHA help
- Fixed playwright version pinned to 1.52.0; mirror fallback to official CDN on failure

Web console file/image support:
- SSE real-time push for images and files via on_event (file_to_send)
- Added /api/file endpoint to serve local files for web preview
- Frontend renders images in media-content container (survives delta/done overwrites)
- File attachment cards with download links; RFC 5987 encoding for non-ASCII filenames

Tool workspace fix:
- Inject workspace_dir as cwd into send and browser tools (previously only file tools)
- Screenshots now save to ~/cow/tmp/ instead of project directory

agent/protocol/agent_stream.py

@@ -300,13 +300,13 @@ class AgentStreamExecutor:
                                     f"with same arguments. This may indicate a loop."
                                 )
                         
-                        # Check if this is a file to send (from read tool)
+                        # Check if this is a file to send
                         if result.get("status") == "success" and isinstance(result.get("result"), dict):
                             result_data = result.get("result")
                             if result_data.get("type") == "file_to_send":
-                                # Store file metadata for later sending
                                 self.files_to_send.append(result_data)
                                 logger.info(f"📎 检测到待发送文件: {result_data.get('file_name', result_data.get('path'))}")
+                                self._emit_event("file_to_send", result_data)
                         
                         # Check for critical error - abort entire conversation
                         if result.get("status") == "critical_error":