Enhance iCalendar export and subscription functionality

[lucs7]

This pull request implements several improvements to iCalendar (ICS) export and subscription functionality. It implements the use of sabre to create the ics feeds and adds calendar naming capabilities.

[Copilot]

Pull request overview

This PR modernizes LibreBooking’s iCalendar (ICS) export/subscription pipeline by switching serialization from a Smarty template to Sabre\VObject, and adds support for naming exported/subscribed calendars (e.g., schedule/resource/group name or localized “My Calendar”).

Changes:

• Replace hand-built ICS template rendering with Sabre\VObject-based VCALENDAR/VEVENT/VAlarm generation.
• Add calendar naming support to subscription/export flow (including resource group name support).
• Update PHPUnit tests and PHPStan config to accommodate Sabre’s magic property access and new serialization expectations.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tpl/Export/ical.tpl	Removes the legacy Smarty ICS template now replaced by Sabre serialization.
Pages/Export/CalendarExportDisplay.php	Implements Sabre\VObject calendar/event/alarm generation and serialization; supports optional calendar name.
Pages/Export/CalendarSubscriptionPage.php	Passes calendar name into the export display renderer.
Pages/Export/SubscriptionPage.php	Stores calendar name on subscription pages and exposes setter.
Pages/Export/ICalendarSubscriptionPage.php	Extends subscription page interface with calendar name setter.
Presenters/CalendarSubscriptionPresenter.php	Determines calendar name (schedule/resource/group/user) and sets it on the page.
lib/Application/Schedule/CalendarSubscriptionService.php	Adds resource-group-name lookup support.
lib/Application/Schedule/iCalendarReservationView.php	Stops pre-escaping RFC5545 TEXT values; relies on Sabre serialization to escape.
tests/Presenters/CalendarExportPresenterTest.php	Updates expectations for raw view values and adds assertions for serialized escaping.
tests/Presenters/CalendarSubscriptionPresenterTest.php	Adds assertions that calendar names are set for schedule/resource/user/group subscription flows.
tests/Application/Schedule/CalendarSubscriptionServiceTest.php	Adds coverage for resource group name lookup behavior.
phpstan.neon	Configures Sabre Node as a universal object crate to avoid false positives on magic ICS properties.
phpstan_next.neon	Same PHPStan adjustment for the “next” ruleset.