Add time tracking (start/stop timer on tasks)

[MeisnerDan]

Add a built-in timer UI to task cards and detail pages. Users can click "Start Timer" to begin time tracking on a task, and "Stop Timer" to end it. Elapsed time is saved to the task's actualMinutes field. This integrates with existing time estimation features and helps users understand work patterns.

Acceptance Criteria

• Start/Stop timer buttons appear on task detail pages and task cards (optional)
• Active timer shows elapsed time with MM:SS format
• Only one task timer can be active at a time (starting a new timer pauses the previous)
• Timer persists across page refreshes (stored in localStorage or task state)
• Stopped timer updates task's actualMinutes field
• Timer pause/resume functionality (not just stop)
• Visual indicator (e.g., badge or highlight) shows which task has active timer
• Historical time entries are logged in a separate timeEntries array on the task
• Unit tests for timer logic and persistence