feat(staged): animate action buttons between enlarged and minimized states

[matt2e]

Summary

• Replaces the separate empty-state action buttons with a single unified set that animates between enlarged (empty timeline) and minimized (populated timeline) states
• Uses CSS transitions for smooth animation of size, spacing, padding, and border properties
• Removes ~60 lines of duplicate empty-state markup and styling by reusing the footer action buttons with conditional enlarged classes

Test plan

• Verify action buttons appear enlarged when the timeline is empty (no commits, notes, or pending items)
• Verify buttons smoothly animate to minimized state when a timeline item is added
• Verify buttons animate back to enlarged state when all items are removed
• Verify hover styles work correctly in both enlarged and minimized states
• Verify disabled state works correctly in both states

🤖 Generated with Claude Code