Local Stopwatch Timer

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This addon provides a basic stopwatch and countdown timer functionality. The popup UI contains start/stop/reset buttons and a display for elapsed time (stopwatch) or remaining time (timer). It uses `setInterval` and `clearInterval` to update the time display. Time values are stored in `sessionStorage` or `chrome.storage.session` to persist across popup closures within the same browsing session. No external synchronization or network requests are involved; timekeeping is entirely based on the browser's internal clock.