Handle missing median time data in metrics

[Timon0512]

Summary

• return None from the median time calculation when no valid values are available
• show clear placeholder messages in the breakout, retracement, and expansion metrics when median times are missing
• ensure accompanying delta text gracefully handles missing numeric values

Testing

• python -m compileall streamlit_app.py

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https://chatgpt.com/codex/tasks/task_e_68e7ce729d388321832be8814bf5a663