Add localStorage persistence and timer reset feature

[Jaswanth1617]

Description

This PR improves the functionality of the Productivity Tracker by implementing persistent storage as well as timer functionality.

Modifications

• Implementation of localStorage for tasks.
• Auto-save functionality of tasks on add, complete, or delete actions.
• Tasks are automatically retrieved when reopening the application.
• Implementation of localStorage for timer.
• Persistent timer value even if page is refreshed.
• Addition of Reset button that sets timer value back to 00:00:00.

Advantages

• Elimination of accidental loss of data after refresh/reload or restarting the browser.
• Enhanced experience of using the product due to persistent timer value.
• Users get the ability to reset their timer easily.
• Does not interfere with existing features; implements new functionality.

Testing

• Add a task, and verify its persistence through refresh.
• Complete a task, check if its state is persisted through reload.
• Delete tasks, make sure that they are gone after refreshing the page.
• Start timer, refresh the page, and make sure timer value is persistent.
• Test if the Reset button works correctly, and timer gets updated accordingly.