perf(app): poll active sessions adaptively (3s active, 30s idle)

[JeanBaptisteRenard]

Problem

The renderer polls active PTY sessions every 3s unconditionally via setInterval(pollActiveSessions, 3000). Each tick performs an IPC round-trip (getActiveSessions) plus a full-sidebar querySelectorAll sweep in updateRunningIndicators() — even when zero sessions are running. On an idle app this is steady, pointless CPU churn.

Fix

Poll adaptively:

• pollActiveSessions() now re-arms itself via scheduleActiveSessionsPoll(), which schedules the next tick at 3s while any PTY is running (activePtyIds.size > 0) and 30s when idle.
• Every renderer code path that starts a session already calls pollActiveSessions() explicitly, so an in-renderer session start re-arms the fast cadence immediately.
• The 30s idle floor still catches sessions started outside the renderer (e.g. scheduler-spawned PTYs) within at most 30s.
• The sidebar timeago setInterval now no-ops when lastActivityTime is empty.

Notes

Behavior-preserving: identical polling work and indicator updates while sessions run; the only change is cadence when idle. Syntax-checked (node --check); the test suite passes.