fix: Display 'No Alarm Scheduled' Message When No Alarm is Set

[keyurgit45]

Description

Expected behaviour

1. If an alarm is scheduled, show the remaining time until it rings.
2. If no alarm is set, display the message "No alarm scheduled" instead of a misleading countdown.

Actual behaviour

1. If an alarm was scheduled, the remaining time until it rang was displayed.
2. If the alarm is turned off, after a few seconds, it will show the remaining time until it rings if it was turned on.

Proposed Changes

await Future.delayed(delay); caused alarms to still notify even after being turned off. To fix this, I swapped the Future for a regular Timer. Now, when an alarm's state changes, the timer is cancelled immediately, ensuring "Next alarm" accurately reflects the alarm's true status.

Fixes #284

Checklist

• Tests have been added or updated to cover the changes
• Documentation has been updated to reflect the changes
• Code follows the established coding style guidelines
• All tests are passing