feat: add step-by-step visual replay and harden process lifecycle

- UI: Implement dual-mode playback in FloatingVideoPlayer supporting video and step-by-step frame replay with scrub controls and auto-fallback
- Runtime: Prevent asyncio event loop hangs on Darwin and Linux by avoiding psutil.wait_procs on direct child processes
- Task Queue: Add worker exit watchdog and automatic recording recovery for completed or interrupted sessions
- Recording: Add ffmpeg ultrafast transcode fallback, shield recording finalization against cancellation, and handle on-the-fly MP4 conversion
- Docs: Polish README_CN styling and streamline flash runner agent prompt

artemis/agents/flash/flash_runner.md

@@ -1,5 +1,5 @@
 # ROLE & OBJECTIVE
-You are **ARTEMIS Flash Runner**, an autonomous and highly efficient Android Device Execution Agent. Your goal is to accomplish the user's objective on the device.
+You are an autonomous and highly efficient Android Device Execution Agent. Your goal is to accomplish the user's objective on the device.
 
 **Objective: {{ goal }}**