v6.15.12

Fixes from @deksden testing (11k sessions):

• Analytics: frontend HTML cache — instant re-render on page revisit (was ~1 min recalc)
• Race condition: leaderboard/analytics won't overwrite current view if user navigated away
• Session count: shows "X with cost data / Y total" to explain gap
• iTerm2: focus now selects correct tab+session (from v6.15.11)