Refactor tasklist cache and sync list handling

[TDannhauer]

Nag: conditional web session refresh for task list changes

Summary

Adopts the Kronolith pattern for keeping the Nag web UI in sync when task lists change via ActiveSync or another session, without reloading prefs and share caches on every page view.

Problem

With horde/nag#25, refreshWebSessionState() ran on every Nag request (prefs->cleanup() / retrieve() plus share list cache expiry). That works but is costly (review comment). List changes from iOS or a second browser tab also need other web sessions to notice without a full logout.

Solution

• Store a lightweight invalidation marker in Horde_Cache (nag.tasklist_cache_ts.<user>), not in user preferences.
• On each web request, compare the marker with tasklist_cache_ts in the PHP session; run the expensive reload only when the marker is newer.
• Call markTasklistCacheChanged() from notifyActiveSyncOfTaskListChange() (always, even if ActiveSync is disabled) and when adding a display-only task list.
• On reload: prune stale display_tasklists / sync_lists, refresh default and sync lists (getSyncLists(true)), persist prefs, update session marker.

ActiveSync device cache behaviour from #25 is unchanged (hierarchy = 0, no prune in notify, collection-only delete).

Files changed

• lib/Nag.php

Test plan

• Open Nag in the browser; normal navigation — no new errors in horde.log.
• With another session or iOS: create, rename, or delete a task list; reload Nag in the browser without logout — lists match, no access errors.
• Confirm ActiveSync folder sync still works (activesync_no_multiplex + sync_lists).