Timer started from task only visible in header badge. Task detail now shows running timer (elapsed, comment) with stop button when active timer belongs to that task.

How to verify

1. Open a task's detail view on an instance with time tracking enabled.
2. In the Time tracking section, click "Start timer".
3. Expected: A "Timer running" row appears in the section with a live elapsed time and a "Stop timer" button.
4. Click "Stop timer".
5. Expected: The row disappears, the header timer badge disappears, and the entry in the list now has an end time and duration.
6. Start a timer from a different task, then open the first task again.
7. Expected: No running-timer row is shown on the first task.