feat(frontend): add Activity page and timeline component styles (#27)

[mitchellecm7]

Summary

Implements the Activity screen with a timeline component for stream lifecycle events as described in issue #27.

Changes

• app/activity/page.tsx — new Activity page with filter tabs (All / Created / Started / Settled / Stopped) and event count badge
• app/components/ActivityTimeline.tsx — new timeline component with color-coded events, empty state, and dark background support
• app/components/ActivityTimeline.test.tsx — 13 tests covering empty state, populated state, and edge cases

Event Types Supported

• Created
• Started
• Settled
• Stopped

Testing

• ✅ Empty state renders correctly
• ✅ Populated state renders all event types
• ✅ Stream name falls back to stream ID when absent
• ✅ Amount and token badge render when provided
• ✅ Timestamps render as semantic <time> elements

Notes

• Readable on dark background with no visual clutter
• Mock events included for development/demo purposes

Closes #27

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[greatest0fallt1me]

@mitchellecm7 Please resolve the conflicts