feat: add target hours selector and update timer calculations based on user input

public/js/state.js

@@ -17,6 +17,7 @@ let timerPausedDuration = 0; // Total paused time in seconds
 let isPaused = false;
 let pauseTimeout = null;
 let currentEntryId = null; // ID of today's entry being timed
+let targetHours = 8; // Target work hours per day (1-10)
 
 // Current month display state
 let displayYear = new Date().getFullYear();