feat: dashboard session layout improvements and approval actions

[castellotti]

Summary

• Tool call approvals in Dashboard: The Allow/Deny/Defer approval bar now appears inline in the Dashboard event feed directly below any pending tool_call_pending event, matching the behavior already present in the Log view.
• 1–2 session vertical fill: Session cards now fully utilize the available vertical space. The event feed was previously capped by the activity chain's width-based capacity; it now renders all events and scrolls, so the card fills its height with real content.
• 3-session 2×2 grid with smart sizing: When exactly 3 sessions are active, the layout switches to a 2×2 grid where the focused session (or the first session when none is focused) spans both rows in the left column at full height, while the other two sessions stack in the right column at half height each. Focus changes dynamically reorder the maximized card.

Test plan

• Trigger a PreToolUse blocking hook and confirm Allow/Deny/Defer buttons appear in the Dashboard event feed
• Open Dashboard with 1 session — card should fill the full height with event rows
• Open Dashboard with 2 sessions — both cards should fill the full height side by side
• Open Dashboard with 3 sessions — verify 2×2 layout with first session spanning full height on the left
• Click a session header to focus it with 3 sessions — verify the focused session becomes the full-height left card