Don't use global TimeZoneRegistry

[rfc2822]

Closes #35.

With this the tests of #32 seem to work fine.

[Copilot]

Pull Request Overview

This PR removes the global TimeZoneRegistry usage and deprecated DateUtils.ical4jTimeZone calls by introducing explicit TimeZoneRegistryFactory.getInstance().createRegistry() usage and passing registries into key functions.

• Replaced global DateUtils.ical4jTimeZone lookups with TimeZoneRegistry.getTimeZone(...) calls.
• Updated APIs (e.g., toIcal4jDateTime, androidifyTimeZone, androidStringToRecurrenceSet) to accept a TimeZoneRegistry parameter.
• Removed deprecated registry helpers and updated all affected tests.

Reviewed Changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
EventValidatorTest.kt	Fixed extension-call syntax and removed unused import
TimeApiExtensions.kt	Added tzRegistry parameter to toIcal4jDateTime
DateUtils.kt	Removed global registry and ical4jTimeZone helper
AndroidTimeUtils.kt	Updated timezone helpers to require TimeZoneRegistry
Multiple *.kt (main & tests)	Replaced DateUtils.ical4jTimeZone imports/calls with registry

Comments suppressed due to low confidence (4)

lib/src/main/kotlin/at/bitfire/synctools/icalendar/ICalendarParser.kt:36

• The KDoc lists a tzRegistry parameter but the parse function signature only accepts reader; please update or remove the outdated @param tzRegistry tag to match the actual signature.

     * @param tzRegistry    time zone registry where VTIMEZONE definitions of the iCalendar will be put

lib/src/main/kotlin/at/bitfire/ical4android/util/AndroidTimeUtils.kt:89

• [nitpick] Defining a new TimeZoneRegistry inside this function on each call can lead to repetitive instantiation; consider passing a shared tzRegistry into the helper or hoisting it to the object scope to reduce duplication.

        val tzRegistry by lazy { TimeZoneRegistryFactory.getInstance().createRegistry() }

lib/src/main/kotlin/at/bitfire/ical4android/util/DateUtils.kt:110

• The parseVTimeZone builder no longer receives a TimeZoneRegistry, which may prevent VTIMEZONE definitions from resolving correctly; restore CalendarBuilder(tzRegistry) or inject a registry instance.

        val builder = CalendarBuilder()

lib/src/main/kotlin/at/bitfire/ical4android/DmfsTask.kt:77

• [nitpick] A per-instance lazy registry works but each task subclass also directly calls createRegistry() elsewhere; consider centralizing registry creation (e.g., in a shared singleton) to avoid scattered instantiation.

    protected val tzRegistry by lazy { TimeZoneRegistryFactory.getInstance().createRegistry() }

[sunkup]

Looks ok. I don't understand why we comment on duration = null instead of deleting it though?