Fixes #698

Problem
In store.js toggleTaskStatus(), the catch block had wrong
revert logic:

task.status = newStatus === 'Done' ? 'Not Started' : 'Done';

This sets the status to the NEW status instead of the
ORIGINAL status on network failure. So if the API call
fails, the task status is not correctly reverted back
to what it was before the toggle.

Fix
Store the original status before toggling:
const originalStatus = task.status;

Then revert to it on network error:
task.status = originalStatus;

Testing

• Toggle task to Done → API succeeds → stays Done ✅
• Toggle task to Done → API fails → reverts to Not Started ✅
• Toggle task to Not Started → API fails → reverts to Done ✅