Add total hours/time spent to /stats dashboard

Feature Request

The /stats dashboard currently shows session count, active days, streaks, and token usage — but doesn't show total time spent in Claude Code.

Motivation

Users want to understand how much time they've invested in Claude Code sessions. Currently the data exists in session JSONL files (first/last timestamps per session), but there's no aggregated view.

Suggested additions to /stats

• Total session time — sum of (last timestamp - first timestamp) per session
• Active time estimate — based on message density / gaps between messages (e.g., exclude gaps > 5 min)
• Average session duration

The longest session duration is already tracked in stats-cache.json, so extending this to total/average duration should be straightforward.

Current workaround

Manually parsing ~/.claude/projects/*//*.jsonl files to extract first and last timestamps per session — but this only works for sessions still on disk.