fix: show time until the event (not the reminder) in the save countdown

[MiMoHo]

Type of change(s)

• Bug fix

What changed and why

When saving an event that has at least one notification reminder, Context.scheduleNextEventReminder() picks the next reminder time (curEvent.getEventStartTS() - curReminder) * 1000L and passes it to Context.scheduleEventIn() as notifyAtMillis. Inside scheduleEventIn, the "time remaining" toast was computed from the notification/alarm time rather than the event start:

val secondsTillNotification = (newNotifyAtMillis - now) / 1000

So the toast counted down to the reminder, not the event. Example: an event 2 hours away with a "10 minutes before" reminder showed "1 hour, 50 minutes" instead of "2 hours".

This changes only the toast computation to use the event start time:

val secondsTillEvent = event.getEventStartTS() - getNowSeconds()

Both getNowSeconds() and event.getEventStartTS() are already imported/used in this file, so no new imports are needed. Scheduling behaviour and the notifyAtMillis < now guard are deliberately left untouched, so the snooze / reschedule path (rescheduleReminder) is unaffected. On this code path secondsTillEvent is guaranteed positive because scheduleNextEventReminder only calls scheduleEventIn when curEvent.getEventStartTS() - curReminder > now with a non-negative reminder offset, so start > now.

Tests performed

• detekt, lint, unit tests and the build all pass locally (CI-equivalent).
• Verified on an Android 15 (API 35) emulator that creating an event with a notification reminder saves correctly and the reminder fires.
• Source-verified: the save-time countdown toast is now computed from the event start (getEventStartTS() - getNowSeconds()) instead of the next reminder time.
• Note: the toast itself is transient (~2s, rendered during the activity-close transition) and could not be frozen into a readable screenshot despite several capture methods, so the exact toast text is verified via CI + code review.

Closes the following issue(s)

• Closes Countdown Message on Event Creation Incorrect #1073

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (verified via CI + source review; see Tests performed).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.

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