fix: resolve exponential timer drain in wake-up challenges by adding …

[vibhutomer]

Description

This PR addresses a critical concurrency bug in alarm_challenge_controller.dart where multiple asynchronous _startTimer() loops could run simultaneously. Previously, calling restartTimer() spawned a new for loop without terminating the old one, leading to an exponential drain on the progress bar and causing users to unfairly fail wake-up challenges.

Proposed Changes

• Introduced a _timerSessionId state variable to track the active timer lifecycle.
• Updated restartTimer() to increment the session ID upon invocation.
• Added a cancellation check (if (currentSessionId != _timerSessionId) break;) inside the _startTimer() async loop. This acts as a cancellation token, instantly killing orphaned loops and ensuring only a single asynchronous loop modifies progress.value at any given time.

Fixes #901

Screenshots

(Not applicable - backend logic and concurrency fix)

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