Add support for time zones when calculating alarm trigger times

[cketti]

Unfortunately, this is a rather large pull request. However, the bulk of it is just moving things around, (hopefully) without changing the behavior. These commits are prefixed with [Refactor].

The functional changes are:

• Extracted the code to calculate the end date using start + duration to Task.end (for calendar events VEvent.getEndDate() was already doing all the work).
• Changed AlarmTriggerCalculator.alarmTriggerToMinutes() (previously ICalendar.vAlarmToMin()) to support time zones when calculating alarm trigger times
• A start time (or end time if the alarm is relative to the end) is now required to return the "alarm minutes". This simplifies the code, but required some changes to tests. (It's also in line with what RFC5545 requires.)

[Copilot]

Pull request overview

Adds time zone–aware alarm trigger minute calculations and centralizes end-time derivation for tasks, aligning alarm handling more closely with RFC5545 requirements and improving DST correctness.

Changes:

• Introduces AlarmTriggerCalculator.alarmTriggerToMinutes() to compute alarm offsets with time zone/DST-aware behavior (including for Period triggers).
• Adds Task.end to derive an effective task end (DUE or DTSTART + DURATION) and updates task alarm mapping to use it.
• Updates calendar/task builders and tests to pass required reference start/end times and validates behavior with new/adjusted test coverage.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
lib/src/main/kotlin/at/bitfire/synctools/util/AlarmTriggerCalculator.kt	New shared implementation for alarm trigger offset calculation with time zone/DST support.
lib/src/main/kotlin/at/bitfire/ical4android/Task.kt	Adds Task.end helper for deriving an effective end date from due/start+duration.
lib/src/main/kotlin/at/bitfire/synctools/mapping/tasks/DmfsTaskBuilder.kt	Switches task alarm minute calculation to AlarmTriggerCalculator and uses task.end.
lib/src/main/kotlin/at/bitfire/synctools/mapping/calendar/builder/RemindersBuilder.kt	Switches event reminder minute calculation to AlarmTriggerCalculator and derives end via getEndDate(true).
lib/src/main/kotlin/at/bitfire/ical4android/ICalendar.kt	Removes the old vAlarmToMin helper now replaced by AlarmTriggerCalculator.
lib/src/test/kotlin/at/bitfire/synctools/util/AlarmTriggerCalculatorTest.kt	New focused unit tests for alarm trigger minute calculation, including DST/time zone scenarios.
lib/src/test/kotlin/at/bitfire/synctools/mapping/calendar/builder/RemindersBuilderTest.kt	Updates tests to include required DTSTART references and validate reminder mapping behavior.
lib/src/test/kotlin/at/bitfire/ical4android/TaskTest.kt	Adds test coverage for Task.end across temporal types and DST boundaries.
lib/src/test/kotlin/at/bitfire/ical4android/ICalendarTest.kt	Removes vAlarmToMin-specific tests that moved to AlarmTriggerCalculatorTest.