Add active usage time histogram

[chrissmartin]

• Ability to see most active hours in the given timeframe. (Could be calls, token count etc..)
• Helps to optimise sessions based on active hours.

[AgentSeal]

Good idea. This fits better in the upcoming web dashboard than the TUI -- the TUI is already dense and we want to keep it focused on actionable data. In the web dashboard it'll be a proper day × hour heatmap with interactive hover. Tracking for that release.