Fix timer: use timestamps instead of setInterval increments

[totomagarinos]

Problem

The study session timer relied on incrementing sessionTime every second using setInterval.
When the tab is inactive, the timer would freeze because browsers throttle background intervals.

Solution

• Store the session start timestamp (startTime) when starting or resuming the session.
• Calculate elapsed time using Date.now() - startTime instead of incrementing by 1 each second.
• Ensures the timer always reflects real elapsed time, even if the tab is inactive.

Testing

• Started a session, left the tab inactive for several minutes, returned, and verified the timer reflects the correct elapsed time.