Build a countdown timer

• Three numeric inputs (hh, mm, ss) set the duration; invalid or negative values are rejected and overflow (for example 90 seconds) is normalised
• Start, Pause and Reset buttons, enabled and disabled according to state (idle, running, paused, finished)
• The display shows hh:mm:ss and updates about every 250 ms or once per second
• The remaining time is computed from timestamps (Date.now() or performance.now() against a target end time), so it does not drift when timers fire late or the tab is throttled
• An onComplete callback fires exactly once when the timer reaches zero, and the timer stops there
• The interval is cleared on pause, reset and unmount

Only after every requirement works.

• Update document.title with the remaining time
• Play a sound or show a notification on completion
• Support several independent timers on one page