Feat(activity): add Recent Activity component to display GitHub events

[KrishKyada]

This PR introduces a "Recent Activity" widget to the dashboard, providing users with a live, chronological feed of their latest GitHub events. This
transforms the dashboard from a static reporting tool into a dynamic productivity hub.

Closes #364

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

Changes Made

• API Route: Implemented /api/metrics/activity to fetch, filter, and format GitHub events (Push, Pull Request, Issues, and Release events).
• Recent Activity Component: Created a new RecentActivity.tsx component featuring a vertical timeline, custom event icons, and human-readable time
  formatting (e.g., "2h ago").
• Multi-Account Support: Integrated with AccountContext to support filtering activity by specific linked GitHub accounts or a combined view.
• Fallback Logic: Added fallback to public events API to ensure the widget remains functional if private event tokens hit limits or specific errors
  occur.
• Dashboard Integration: Added the widget as a full-width section at the bottom of the main dashboard.
• Loading & Empty States: Implemented polished skeleton loaders and clear empty/error state handling.

How to Test

1. Sign in to the application and navigate to the Dashboard.
2. Scroll to the bottom of the page to find the Recent Activity widget.
3. Verify that your latest GitHub actions (commits, PRs, etc.) are listed with correct titles and timestamps.
4. Click an item: Ensure it opens the corresponding repository or event on GitHub in a new tab.
5. Switch accounts: Use the account toggle to verify that the activity list filters correctly for different linked accounts.
6. Refresh: Click the "Refresh" button to manually trigger an update of the feed.

Screenshots

Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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