fix: reset timePassed when timer duration is changed in study timer

[akhilmodi29]

Fixes #696

Problem
When a user changes the timer duration after pausing mid-session,
timePassed was never reset to 0. This caused:

• Restarting timer after duration change uses stale timePassed
• Timer shows wrong remaining time
• Timer ends earlier than expected duration

Fix
Added timePassed = 0 inside the timerDurationInput change
handler so elapsed time is always reset when duration changes.
Also removed the timePassed === 0 condition since we now
always reset it on duration change.

Testing

• Start timer for 25 mins, pause at 5 mins ✅
• Change duration to 10 mins ✅
• Restart timer — shows correct 10 mins remaining ✅
• Normal timer flow still works correctly ✅

[akhilmodi29]

@Charushi06 Please review this PR and add the required labels including gssoc:approved. This fixes the timer timePassed not reset bug in study timer.

Labels suggested:

• level:intermediate
• type:bug
• quality:clean