Add long break (15 min) after every 4 Pomodoro sessions

[anoopcodehack]

File: client/src/hooks/usePomodoro.js

Real Pomodoro technique = 25min work → 5min break × 4,
then a 15min long break.

Current code just alternates work/break forever with no long break.

Fix: when sessionCount % 4 === 0 && sessionCount > 0,
use LONG_BREAK_DURATION (15 * 60) instead of BREAK_DURATION