Fix #5331: update system-wide alert preview countdown dynamically

[guillermo-escire]

References

Fixes #5331

Description

Fixes the system-wide alert countdown timer in the preview. The countdown now updates dynamically every second and disappears when it reaches zero.

Instructions for Reviewers

Changes included in this PR:

• Added checks for date and time before updating the preview to prevent errors.
• Modified the allocateTimeUnits function so that when the countdown reaches zero, days, hours, and minutes are set to 0 and the visible countdown stops.
• Adjusted the previewSubscription using interval(1000) to refresh the countdown every second.
• Added a comment to the previewSubscription variable for consistency with other variables (counterEnabled$, previewMinutes, previewHours, previewDays).

How to test:

1. Go to System Alerts → System-Wide Alert.
2. Activate an alert and enable the countdown timer.
3. Set the timer to one minute in the future.
4. Observe the alert preview: the countdown should update dynamically every second showing the correct remaining time.
5. When it reaches zero, the counter disappears from the banner.

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