Modernize Workout around live sessions

[hamzamerzic]

Summary

• Refocus the Session tab around active workout logging, with a distinct ready state, contextual finish controls, and a phone-sized Add activity dock.
• Remove coach and workout-planning prompts while preserving optional natural-language logging.
• Align the header and actions with other Möbius apps, including clearer History controls and a larger chat affordance.
• Rework Insights around recent training, activity variety, plain-language records, and a more compact progress overview.
• Keep the CAS integrity harness compatible with the current runtime hook contract.

Testing

• npm test (99 passing)
• App validation passed with no errors or warnings.
• Verified Session, Add activity, History, Insights, and chat behavior at desktop and phone sizes.