feat(nag): ActiveSync recurring task instance completion

[TDannhauer]

Summary

• Sync recurring task instance completion, un-completion, and series-master updates from ActiveSync clients (iOS, Outlook) to Nag without spawning duplicate rows
• Guard getNextDue() null dereferences in the task detail view and portal Summary block
• Add the missing Nag::isTaskViewFilter() helper used when building complete-link return URLs

Motivation

ActiveSync clients complete individual recurrence instances differently from the web UI. Nag previously created duplicate task rows or failed to persist per-instance completion state. The follow-up fixes address runtime errors introduced while wiring list-return URLs and next-due display for recurring tasks.

Changes

• Nag_Task::fromASTask(): route dead-occurrence Adds and master Modifies to dedicated recurrence handlers; track completions[] instead of toggling series completion
• Nag_Api::replace() / import(): resolve the series master for instance messages and merge dead-occurrence Adds into the master UID
• Nag_Driver::modifyFromHash(): persist recurrence/completion state without toggleComplete() side effects
• Preserve the total RRULE COUNT on master Modify (clients send the remaining-occurrence count, not the series total)
• Guard null getNextDue() in templates/view/task.inc and lib/Block/Summary.php
• Add Nag::isTaskViewFilter() for Nag::VIEW_* tab/filter validation in Nag_Task::process()

Test plan

• vendor/bin/phpunit --bootstrap vendor/autoload.php vendor/horde/nag/test/Nag/Unit/Task/ActiveSyncRecurrenceTest.php (29 tests)
• Complete a single recurrence instance on iOS; verify Nag keeps one master task with updated due date/completion state
• Un-complete a completed instance on iOS; verify the instance reopens without duplicate rows
• Open/edit a task from the Nag list view tabs (All, Incomplete, Future, Completed); verify no fatal error
• Complete a task from the list view and confirm return URL preserves the active filter tab