Activity feed: Add collapsible grouping for similar events

[lbedner]

Summary

Add subtle toggle controls to the Activity feed that allow users to switch between:

1. Expanded view - Show all individual events (current behavior)
2. Collapsed/grouped view - Roll up similar events into grouped entries

Problem

The Activity feed can become cluttered with repetitive events, especially for services that flap between states (e.g., Cache going healthy → unhealthy → healthy multiple times). This makes it harder to scan for meaningful activity.

Proposed Solution

• Group similar events by type/source (e.g., all Cache state changes roll into one "Cache: 8 state changes" entry)
• Use familiar UI patterns so users "just get it" - think GitHub's commit squashing, or how notification systems group similar alerts
• Keep the toggle small and unobtrusive - this is a power-user feature, not a primary action
• Clicking a grouped item expands to show the individual events

Design Considerations

• Lean, subtle triggers - not prominent buttons
• Follow familiar patterns from other tools (GitHub, Slack notifications, etc.)
• Default could be expanded or collapsed - TBD based on UX preference
• Preserve the dropdown chevron for individual event details

Context

Screenshot showing repetitive Cache status changes that would benefit from grouping.